Add some commands to run Coverage and Checks locally

[mtbottle]

This change does 2 things

1. Adding a Makefile to run checks and create a coverage report
2. Add some tests to a random file (cache)

This is a first step to getting us higher test coverage to prepare Contile for Rapid Release.

[pjenvey]

I'll note there's also the cargo-llvm-cov crate now which appears to be a maybe more convenient alternative to grcov. It's been suggested it works better/is more accurate but I'm wondering if that's due to it forcing RUST_TEST_THREADS=1 to avoid a known issue w/ -Cinstrument-coverage If we're seeing intermittent differences in coverage w/ grcov also we'll probably need to force it as well.

[mtbottle]

I've switched to using cargo-llvm-cov because I think it's simpler and cleaner to use.

[ncloudioj]

LGTM, thanks!