coverage: Add #[rustfmt::skip] to tests with non-standard formatting

These tests deliberately use non-standard formatting, so that the line
execution counts reported by `llvm-cov` reveal additional information about
where code regions begin and end.

tests/coverage/while_early_ret.coverage

@@ -1,6 +1,7 @@
    LL|       |#![allow(unused_assignments)]
    LL|       |// failure-status: 1
    LL|       |
+   LL|       |#[rustfmt::skip]
    LL|      1|fn main() -> Result<(), u8> {
    LL|      1|    let mut countdown = 10;
    LL|       |    while