Better watcher mode

[jzaefferer]

• cancel previous run when triggering a new one, probably using subprocesses. Maybe using nice or similar tools to set lower CPU priority
• allow triggering a manual run (with Enter or any key), without file changes happening. Good when making changes in der underlying app (Re-run tests on pressing enter in watch mode #289)
• maybe detect the app reloading itself, to trigger another run
• better cpu utilisation: number of CPUs - 1? Reduce CPU concurrency in watch mode #306
• dynamic headless/non-headless option: run initial selection headless, then rerun failed tests non-headless for debugging in Chromium - This is too expensive. Add support for toggling debug mode during watch #305 adds a way to toggle debug mode during watch

Not directly related to watcher, but would make a good package:

• interactive test selection (by name, by regex), similar to Jest Add interactive pattern search to watch mode #303
• option to only run failed; when all passed, go back to previous filter/selection
• presets of test selection per environment - "local frontend" runs frontend_* tests by default. Better than running all tests by default, can still be overridden