Running tests locally

[eginhard]

The contribution guidelines recommend to run make test "to see how your updates work with the rest of the project". This doesn't seem to be a useful recommendation because many tests involve training models and synthesising data and might take hours to run. The zoo tests also download many models and then completely remove the user's $TTS_HOME folder, which is unexpected.

Is there some subset of fast unit-tests that make sense to run locally and could be recommended there instead, leaving the rest to CI?