[Tracking] High-impact flaky test issues

[gjoseph92]

We have lots of flaky tests. But in a lot of cases, many different tests are all failing in the same way. Here are "high impact" classes of CI failures:

1. 13 different tests: Flaky tests: OSError: Timed out trying to connect to tcp://127.0.0.1:8786 after 5 s #6731, likely includes Flaky tests: plain TimeoutError during client connection to scheduler #6757 for 2 more
2. 6 different tests: Flaky tests: coroutine 'InProc.write' was never awaited #6551
3. 2 tests: Flaky tests: ResourceWarning: unclosed cluster SSHCluster #6771
4. 2 at least: Flaky tests: assert False, (bad_thread, call_stacks) - Worker executor thread still running after Nanny.kill #6796