By moving `block_on` to an auxiliary crate, we avoid having to keep a separate
copy of it in every async test.
(This also incorporates some small tweaks to the headers in `await_ready.rs`.)
For coverage tests, splitting code across multiple lines often makes the
resulting coverage report easier to interpret, so we force rustfmt to retain
line breaks by adding dummy line comments with `//`.
Without the workaround applied, this test will produce malformed mappings that
cause `llvm-cov` to fail.
(And if it does emit well-formed mappings, they should be obviously incorrect.)
