Fail AzDO Pipeline if number of passed tests fall below a certain threashold.

### Is your feature request related to a problem? Please describe.

This week, the number of passed tests as part of our daily CI builds and PR checks this week decreased from [~85,000](https://dev.azure.com/dnceng/public/_build/results?buildId=758545&view=ms.vss-test-web.build-test-results-tab) to [~20,000](https://dev.azure.com/dnceng/public/_build/results?buildId=760441&view=ms.vss-test-web.build-test-results-tab). Despite this, the PR checks passed an the build stayed green.



The tests stopped running because of an issue with xunit/arcade (see https://github.com/dotnet/arcade/pull/5908) that caused `dotnet test` runs to fail silently, but I think automated checks should have caught the dramatic reduction in the number of passed tests.

### Describe the solution you'd like

We should set some sort of minimum threshold on the number of passed tests for a CI build/PR check pass. If we ever intentionally reduce the number of tests we run intentionally, we can manually adjust this threshold.