Change TestOpWaitiness to use MultiProcessTestCase (#121046)
The test has been failing sporadically rencetly in CI and the failures
are not reproducible locally, likely due to some nasty race conditional
related a combination of MultiThreadedTestCase, the use of global state
and finalizers, and the recently introduced test decorator for native
funcol migration.
Switching to the test to use MultiProcessTestCase to provide better
isolation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121046
Approved by: https://github.com/weifengpy
