[AzureDevOpsTests] Refactor unit tests to improve stability (#2454)

* Correct regex generation logic

* Refactor variable name

* Mock Azure DevOps test result summary API

* Add live tests

Updated mocked tests to use expected values for assertion. Added live
tests to test the API. Added `no tests` as an acceptable result for live
tests.

* Declare common nock setup functions to avoid repetition