Improving worker shutdown test to prevent regressions

[brettsam]

Test-only change.

In #9738, I fixed a race condition on worker shutdown. However, the test I wrote was very hard to use as a repro and thus was not a great regression test. I've since found a better way to time the multiple threads.

Currently submitting this back when the bug existed to ensure it would have caught the regression.
Edit: This is now ready.

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• I have added all required tests (Unit tests, E2E tests)

[brettsam]

/azp run

[azure-pipelines]

Pull request contains merge conflicts.

[brettsam]

For one of the steps here I reverted the fix and let the test run to ensure we would have caught it.

Proof that the test failed in CI before the fix was applied: https://azfunc.visualstudio.com/Azure%20Functions/_build/results?buildId=158816&view=ms.vss-test-web.build-test-results-tab&runId=1371853&resultId=101182&paneView=debug