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/simple_loop.coverage

@@ -1,5 +1,6 @@
    LL|       |#![allow(unused_assignments)]
    LL|       |
+   LL|       |#[rustfmt::skip]
    LL|      1|fn main() {
    LL|      1|    // Initialize test constants in a way that cannot be determined at compile time, to ensure
    LL|      1|    // rustc and LLVM cannot optimize out statements (or coverage counters) downstream from