Fix task parallel processor test

[yycptt]

What changed?

• Fix flaky test TestSubmitProcess_Stopped_FailSubmission

Why?

• There's no guarantee that calling Submit() after Stop() will make the task get rescheduled immediately since worker goroutine shutdown is async. It's possible that a worker didn't get the shutdown notification immediately after Stop() is called and picks up the submitted task from task channel.
• Recent changes to parallel processor makes worker goroutine shutdown slightly slower than before (shutdown notification is propagated via another worker monitor goroutine). and makes ^ race condition in test visible.

How did you test it?

go test -run TestParallelProcessorSuite -testify.m TestSubmitProcess_Stopped_Submission -count=10000 -v

Potential risks

Is hotfix candidate?