[WIP] Fix Cythonized Scheduler test failures

[jakirkham]

Reverts the tests skipped/xfailed in PR ( #4764 ) due to issues encountered with the Cythonized Scheduler. Doing this to fix those tests and get them passing in the Cythonized Scheduler case as well.

Note: This PR mainly exists for tracking purposes while we sort out the different tests failures.

• Closes #xxxx
• Tests added / passed
• Passes black distributed / flake8 distributed / isort distributed

[jakirkham]

Was able to fix a couple of these without too much difficulty. Broke out in PR ( #4768 ).

[jakirkham]

One avenue to addressing these remaining issues would be to continue with splitting apart Scheduler & SchedulerState with Martin's PR ( #5176 ), which opens the door to moving SchedulerState into its own module. If SchedulerState could live in its own module that gets Cythonized without the Scheduler, there would be less need to worry about how Cython handles some of these other test failures that cropped up with the Scheduler.