Unittest discovery silently fails when Pytest is not installed

Testing #13301

Run Python: Configure Tests
Choose unittest
🐛 nothing happens. When I open the test view I see that there was a discovery error and that I should see Output > Python. I would expect an error message with a link or button to open the output channel
🐛 the output channel says test discovery failed because pytest wasn't installed. Should we help the user install pytest in this flow? Also, is it expected that pytest is required even if I'm trying to use unittest?

Provide feedback