Add test setup fixture

[jrbourbeau]

This PR adds a new initialize_test fixture which is automatically applied to each test in our test suite. This fixture does a few things:

1. Disable custom preloads in tests, which can interact with tests in unexpected ways
2. Ensures Dask's configuration system is reset for each test
3. Restores default logging levels for each test

I should note that points 2 and 3 above were previously being done in our clean content manager (so all tests using gen_cluster, gen_test, or cleanup). This PR just broadens their reach to all tests.

Closes #5240

cc @mrocklin

[jrbourbeau]

The changes here have caused some tests to start failing. Interestingly if I run distributed/tests/test_client.py::test_default_get or distributed/tests/test_client.py::test_Future_exception_sync_2 individually, they pass. However, if I run the entire distributed/tests/test_client.py module, then those two tests fail. This might have uncovered some unsound logic that was previously going unnoticed...

EDIT: Nvm, the test failures were due to updating the Dask config with the original values without completely clearing it beforehand. I've pushed up a fix

[jrbourbeau]

cc @ian-r-rose if you get a chance to take a look at this

[fjetter]

Love it

[jrbourbeau]

Thanks for reviewing @fjetter!