Add `autoTestDiscoverOnSavePattern` setting to filter when discovery is run

[karthiknadig]

[karthiknadig]

Verification steps:

1. Open a python project with tests written using pytest
2. Ensure tests are discovered. Making changes to any python file should trigger discovery.
3. Add python.testing.autoTestDiscoverOnSavePattern and use glob pattern to filter out some files.
4. Only editing those files should trigger discovery.

[roblourens]

You might look at our files.exclude / search.exclude settings for a different structure. It's an object with glob keys which lets you layer the setting, turning off or on individual patterns at different settings scopes. Like I might want to set some glob for autoTestDiscoverOnSavePattern in my user settings, and another one in my workspace settings, and have both applied?