test(frontmatter): Rename tests to make coverage more obvious

When working on the stabilization report,
I found it annoying to determine what cases were covered because areas
of the frontmatter feature were either not in the file name or in
inconsistent locations.

This moves the area of frontmatter to the start of the file name and the
moves to more specific the later in the file name so coverage is easier
to see.

tests/ui/frontmatter/fence-close-extra-after.stderr

@@ -1,5 +1,5 @@
 error: extra characters after frontmatter close are not allowed
-  --> $DIR/extra-after-end.rs:2:1
+  --> $DIR/fence-close-extra-after.rs:2:1
    |
 LL | ---cargo
    | ^^^^^^^^

tests/ui/frontmatter/fence-indented-mismatch.stderr

@@ -1,5 +1,5 @@
 error: unclosed frontmatter
-  --> $DIR/frontmatter-whitespace-2.rs:1:1
+  --> $DIR/fence-indented-mismatch.rs:1:1
    |
 LL | / ---cargo
 ...  |
@@ -7,13 +7,13 @@ LL | |
    | |_^
    |
 note: frontmatter opening here was not closed
-  --> $DIR/frontmatter-whitespace-2.rs:1:1
+  --> $DIR/fence-indented-mismatch.rs:1:1
    |
 LL | ---cargo
    | ^^^
 
 warning: use of a double negation
-  --> $DIR/frontmatter-whitespace-2.rs:9:6
+  --> $DIR/fence-indented-mismatch.rs:9:6
    |
 LL |     ---x
    |      ^^^

tests/ui/frontmatter/fence-indented.stderr

@@ -1,17 +1,17 @@
 error: invalid preceding whitespace for frontmatter opening
-  --> $DIR/frontmatter-whitespace-1.rs:1:1
+  --> $DIR/fence-indented.rs:1:1
    |
 LL |   ---
    | ^^^^^
    |
 note: frontmatter opening should not be preceded by whitespace
-  --> $DIR/frontmatter-whitespace-1.rs:1:1
+  --> $DIR/fence-indented.rs:1:1
    |
 LL |   ---
    | ^^
 
 error: unclosed frontmatter
-  --> $DIR/frontmatter-whitespace-1.rs:1:3
+  --> $DIR/fence-indented.rs:1:3
    |
 LL | /   ---
 LL | |
@@ -21,7 +21,7 @@ LL | |
    | |_^
    |
 note: frontmatter opening here was not closed
-  --> $DIR/frontmatter-whitespace-1.rs:1:3
+  --> $DIR/fence-indented.rs:1:3
    |
 LL |   ---
    |   ^^^

tests/ui/frontmatter/fence-mismatch-1.stderr

@@ -1,5 +1,5 @@
 error: frontmatter close does not match the opening
-  --> $DIR/mismatch-1.rs:1:1
+  --> $DIR/fence-mismatch-1.rs:1:1
    |
 LL |   ---cargo
    |   ^--

tests/ui/frontmatter/fence-mismatch-2.stderr

@@ -1,5 +1,5 @@
 error: frontmatter close does not match the opening
-  --> $DIR/mismatch-2.rs:1:1
+  --> $DIR/fence-mismatch-2.rs:1:1
    |
 LL |   ----cargo
    |   ^---
@@ -13,7 +13,7 @@ LL | | ---cargo
    |   ...while the close has 3 dashes
 
 error: extra characters after frontmatter close are not allowed
-  --> $DIR/mismatch-2.rs:3:1
+  --> $DIR/fence-mismatch-2.rs:3:1
    |
 LL | ---cargo
    | ^^^^^^^^

tests/ui/frontmatter/fence-unclosed-1.stderr

@@ -1,5 +1,5 @@
 error: unclosed frontmatter
-  --> $DIR/unclosed-1.rs:1:1
+  --> $DIR/fence-unclosed-1.rs:1:1
    |
 LL | / ----cargo
 ...  |
@@ -7,7 +7,7 @@ LL | |
    | |_^
    |
 note: frontmatter opening here was not closed
-  --> $DIR/unclosed-1.rs:1:1
+  --> $DIR/fence-unclosed-1.rs:1:1
    |
 LL | ----cargo
    | ^^^^

tests/ui/frontmatter/fence-unclosed-2.stderr

@@ -1,5 +1,5 @@
 error: unclosed frontmatter
-  --> $DIR/unclosed-2.rs:1:1
+  --> $DIR/fence-unclosed-2.rs:1:1
    |
 LL | / ----cargo
 ...  |
@@ -8,13 +8,13 @@ LL | | }
    | |__^
    |
 note: frontmatter opening here was not closed
-  --> $DIR/unclosed-2.rs:1:1
+  --> $DIR/fence-unclosed-2.rs:1:1
    |
 LL | ----cargo
    | ^^^^
 
 error[E0658]: frontmatters are experimental
-  --> $DIR/unclosed-2.rs:1:1
+  --> $DIR/fence-unclosed-2.rs:1:1
    |
 LL | / ----cargo
 ...  |

tests/ui/frontmatter/fence-unclosed-3.stderr

@@ -1,17 +1,17 @@
 error: invalid preceding whitespace for frontmatter close
-  --> $DIR/unclosed-3.rs:12:1
+  --> $DIR/fence-unclosed-3.rs:12:1
    |
 LL |     ---x
    | ^^^^^^^^
    |
 note: frontmatter close should not be preceded by whitespace
-  --> $DIR/unclosed-3.rs:12:1
+  --> $DIR/fence-unclosed-3.rs:12:1
    |
 LL |     ---x
    | ^^^^
 
 error: frontmatter close does not match the opening
-  --> $DIR/unclosed-3.rs:1:1
+  --> $DIR/fence-unclosed-3.rs:1:1
    |
 LL |   ----cargo
    |   ^---
@@ -26,13 +26,13 @@ LL | |     ---x
    |   ...while the close has 3 dashes
 
 error: extra characters after frontmatter close are not allowed
-  --> $DIR/unclosed-3.rs:12:1
+  --> $DIR/fence-unclosed-3.rs:12:1
    |
 LL |     ---x
    | ^^^^^^^^
 
 error: unexpected closing delimiter: `}`
-  --> $DIR/unclosed-3.rs:15:1
+  --> $DIR/fence-unclosed-3.rs:15:1
    |
 LL | }
    | ^ unexpected closing delimiter

tests/ui/frontmatter/fence-unclosed-4.stderr

@@ -1,5 +1,5 @@
 error: unclosed frontmatter
-  --> $DIR/unclosed-4.rs:1:1
+  --> $DIR/fence-unclosed-4.rs:1:1
    |
 LL | / ----cargo
 LL | |
@@ -7,7 +7,7 @@ LL | |
    | |_^
    |
 note: frontmatter opening here was not closed
-  --> $DIR/unclosed-4.rs:1:1
+  --> $DIR/fence-unclosed-4.rs:1:1
    |
 LL | ----cargo
    | ^^^^

tests/ui/frontmatter/fence-unclosed-5.stderr

@@ -1,5 +1,5 @@
 error: unclosed frontmatter
-  --> $DIR/unclosed-5.rs:1:1
+  --> $DIR/fence-unclosed-5.rs:1:1
    |
 LL | / ----cargo
 ...  |
@@ -7,13 +7,13 @@ LL | |
    | |_^
    |
 note: frontmatter opening here was not closed
-  --> $DIR/unclosed-5.rs:1:1
+  --> $DIR/fence-unclosed-5.rs:1:1
    |
 LL | ----cargo
    | ^^^^
 
 error[E0658]: frontmatters are experimental
-  --> $DIR/unclosed-5.rs:1:1
+  --> $DIR/fence-unclosed-5.rs:1:1
    |
 LL | / ----cargo
 ...  |

tests/ui/frontmatter/fence-unclosed-6.stderr

@@ -1,5 +1,5 @@
 error: unclosed frontmatter
-  --> $DIR/unclosed-6.rs:1:1
+  --> $DIR/fence-unclosed-6.rs:1:1
    |
 LL | / ---
 LL | |
@@ -10,7 +10,7 @@ LL | |
    | |_^
    |
 note: frontmatter opening here was not closed
-  --> $DIR/unclosed-6.rs:1:1
+  --> $DIR/fence-unclosed-6.rs:1:1
    |
 LL | ---
    | ^^^

tests/ui/frontmatter/infostring-comma.stderr

@@ -1,5 +1,5 @@
 error: invalid infostring for frontmatter
-  --> $DIR/infostring-fail.rs:1:4
+  --> $DIR/infostring-comma.rs:1:4
    |
 LL | ---cargo,clippy
    |    ^^^^^^^^^^^^

tests/ui/frontmatter/infostring-dot-leading.stderr

@@ -1,5 +1,5 @@
 error: invalid infostring for frontmatter
-  --> $DIR/dot-in-infostring-leading.rs:1:4
+  --> $DIR/infostring-dot-leading.rs:1:4
    |
 LL | ---.toml
    |    ^^^^^

tests/ui/frontmatter/infostring-hyphen-leading.stderr

@@ -1,5 +1,5 @@
 error: invalid infostring for frontmatter
-  --> $DIR/hyphen-in-infostring-leading.rs:1:4
+  --> $DIR/infostring-hyphen-leading.rs:1:4
    |
 LL | --- -toml
    |    ^^^^^^

tests/ui/frontmatter/infostring-space.stderr

@@ -1,5 +1,5 @@
 error: invalid infostring for frontmatter
-  --> $DIR/space-in-infostring.rs:1:4
+  --> $DIR/infostring-space.rs:1:4
    |
 LL | --- cargo clippy
    |    ^^^^^^^^^^^^^

tests/ui/frontmatter/location-after-tokens.stderr

@@ -1,5 +1,5 @@
 error: expected item, found `-`
-  --> $DIR/frontmatter-after-tokens.rs:3:1
+  --> $DIR/location-after-tokens.rs:3:1
    |
 LL | ---
    | ^ expected item