[Draft] Use VSCode test system

[GeorchW]

As discussed in #272, there's now an official way to write test extensions. I'm working on porting this extension.

TODO:

• Test Discovery
• Test Execution
• Debugging
• Display enqueued/started tests
• Watch
• Diagnostics for failed tests
• Jump to test definition
• Use proper file structure instead of dumping it all into extension.ts :-)
• Remove old UI code
• Ensure that the old triggers for test discovery still work
• Ensure that code lens still works
• Ensure that status bar item still works
• Ensure that skipped tests are handled appropriately
• Ensure that the extension tests are working

[mikethea1]

@GeorchW did you ever end up porting this extension?

[GeorchW]

No, I haven't touched this in years - I'm not doing any .NET development right now. IIRC, there were plans to support testing in one of the "official" C#/.NET extensions for VS Code, but I can't remember which one or where it was. Anyway, that kinda stopped my motivation, along with me having less time probably.

Feel free to use my unfinished work however you like if you're into that