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/closure_bug.rs

@@ -3,6 +3,7 @@
 // the coverage report. However, an unstable sort was causing them to be treated
 // inconsistently when preparing coverage spans.
 
+#[rustfmt::skip]
 fn main() {
     let truthy = std::env::args().len() == 1;