Expand description
Tidy check to ensure #[test] and #[bench] are not used directly inside
core or alloc.
core and alloc cannot be tested directly due to duplicating lang items.
All tests and benchmarks must be written externally in
{coretests,alloctests}/{tests,benches}.
Outside of core and alloc, tests and benchmarks should be outlined into
separate files named tests.rs or benches.rs, or directories named
tests or benches unconfigured during normal build.