Perform test discovery per-file when a file is updated/added.

[ericsnowcurrently]

Currently we only perform test discovery on the whole project. This is unnecessary when discovery happens in response to a file being created, updated, or deleted. We should be able to perform discovery on a per-file basis in those cases. Doing so will be more efficient and allow us to be more granular in how we present test status.

This is effectively blocked by a uniform test discovery API for the different test frameworks (e.g. like we're doing in #3911 for pytest).

[Conchylicultor]

@luabud thank you for looking into this !!

When implementing per-file discovery, it would be awesome if all discovered tests gets displayed, even if other files fails. Currently a single wrong file cause the entire test discovery to fail. (See #13301 for context)