Document how to run unit tests

[stefnotch]

It'd be lovely if potential contributors could run the unit tests that rely on loom. Currently, I do not know how to run those.

RUSTFLAGS="--cfg loom" cargo test --release does not seem to work

[nagisa]

Yeah, unfortunately the test suite sucks quite a bit due to the fact that they rely on tracy to operate and mocking this component out and/or testing with it is Extremely Inconvenient -- it really isn't meant to be used in automated contexts like these.

Maybe one day I'll figure out some work-around to this but today is not that day :/