Watch mode: Run only those tests that are impacted by file change

[nightscape]

If I understand correctly, the result of the incremental Zinc compilation should yield the information which test files have been impacted by a change in another test or implementation file.
If so, this information could be used to trigger a re-run of only those impacted test files in watch mode, giving nice short feedback cycles even in bigger projects.
In SBT I would never dare to touch the codebase to do such a change, but in Mill I would dare to give it a try 😃

[nightscape]

Probably related: #2417

[lefou]

Nobody is going to stop you. 👌‍ If I'm not mistaken, sbt has some test task variant, that is also only running tests affected by change. testQuick or something like that. Don't know if it is the same, but it might be worth to get inspiration from it.