Set overall timeout on unit test jobs

[dsherry]

https://stackoverflow.com/a/59076067

If we set a timeout on the overall runtime of the unit tests, that could be helpful in preventing us from adding a bunch of short-running tests which cumulatively add up to a long runtime (i.e. via pytest.mark.parametrize).

@freddyaboulton recently merged a PR to have a per-test timeout set which is helpful.