test(frontmatter): Check handling of long code fences

tests/ui/frontmatter/fence-too-many-dashes.rs

@@ -0,0 +1,12 @@
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+// ignore-tidy-linelength
+[dependencies]
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+
+#![feature(frontmatter)]
+
+//@ check-pass
+
+// check that we limit fence lengths
+
+fn main() {}