Implement pytest plugin for test discovery/execution.

[karthiknadig]

The solution should address following issues, in addition to the ones covered by prototype requirements:

Problems with test id:

• Folders named 'py' result in "Test result not found" #17414
• VSCode not running discovered PyTest tests when path contains a symlink #17573
• "Test result not found" if cwd is a subdirectory in the workspace #18030

Problems with other plugins or python or libs printing to output:

• Pytest discovery stuck in Output panel #18656

Once these are validated close them against the PR for this issue.

[eleanorjboyd]

closing this issue and moving all this information #17242 to allow for one source of truth around the pytest rewrite.