Attempt to stabilize flaky tests

[alzimmermsft]

Description

Attempts to stabilize some flaky tests.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[anuchandy]

Nice to see a different concurrent Collection type solves the problem. I don’t understand what special about CopyOnWriteArrayList caused the failure, maybe test assertion is getting not up to date iterator to the list-snapshot. But regardless, for the UT, light weight ConcurrentQueue seems to be good enough.

[trask]

thx @alzimmermsft!