sleeps or poll in tests need to centrally regulated

[robnagler]

When pykern test is run in parallel, delays occur. Certain tests, e.g. supervisor_purge_free_sims_test.py, have sleeps to "ensure" that something happens. This particular test can't poll runStatus, because this puts the job back in ComputeJob.instances, and the sim won't get purged. Other tests may be less sensitive. job_cancel_test.py polls runStatus 5 times with 1 second sleeps to see that the job is running, canceled, etc.

Need some centralization on sleeps for tests that can't poll, and for tests that can, they should.