Show message when trying to run a test that hasn't been discovered #3631
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Motivation
Closes #3628
If users place tests in directories that don't match our expected glob pattern convention, the files will not be discovered, but the code lenses might still be available. That will currently cause the buttons to do nothing with no feedback, which is not great.
Implementation
Started showing a message explaining why a test might have not been discovered, added a new note to our docs and linked to it.