coverage: Tweak individual tests to be unaffected by rustfmt

Some of these tests use non-standard formatting that we can simulate by
strategically adding `//` line comments.

One contains `where` clauses that would be split across multiple lines, which
we can keep on one line by moving the bounds to the generic type instead.

tests/coverage/while.coverage

@@ -1,6 +1,7 @@
    LL|      1|fn main() {
    LL|      1|    let num = 9;
    LL|      1|    while num >= 10 {
+   LL|      0|        // loop body
    LL|      0|    }
    LL|      1|}