[tooling/CI] Extend mock_uss startup timeout, separate unit tests

[BenjaminPelletier]

This PR attempts to address two CI issues I've seen recently:

1. Sometimes tests spuriously fail (and succeed when retried without any changes) because the suite of mock_uss instances doesn't become healthy in time. This PR extends the startup timeout from 60 seconds to 90 seconds.
2. Unit tests are currently being run in a job named "hygiene". When this job fails, it is misleading where to look for the problem. This PR separates unit tests into their own job and keeps hygiene as hygiene-only (often fixable with make format).