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/async.rs

@@ -1,6 +1,8 @@
 #![feature(coverage_attribute)]
+#![feature(custom_inner_attributes)] // for #![rustfmt::skip]
 #![feature(noop_waker)]
 #![allow(unused_assignments, dead_code)]
+#![rustfmt::skip]
 // edition: 2018
 // compile-flags: -Copt-level=1