Make test discovery cancellable

[karthiknadig]

Currently, the auto-discovery run is not cancellable. This can some times get in the way when say pytest is not installed , and you install pytest it can prevent from running immediately. May be, we can check if pytest exists or if path is provided, if eithe is true then run discovery using withProgress API so even the auto discover can be cancelled.

[roblourens]

I see progress on the view, but nothing that I can use to cancel discovery? Maybe it's too fast? What should I see?

[eleanorjboyd]

it should work with very repos that have a very large number of tests - I know https://github.com/pypa/packaging has a lot. Maybe someone from the python team with a large repo could test? @karthiknadig @anthonykim1 @luabud @cwebster-99

[karthiknadig]

The repro is enable autosave with short time duration between saves or trigger a lot of saves while making changes. Then make random edits to the files. For a repo like packaging, it should take a few seconds to finish discovery. So, with these changes, you may see it hang.