Closes https://github.com/microsoft/playwright/issues/22211
Currently, when the server notifies the UI about changed files, the UI
determines what files to re-run based on an old test list. By listing
tests before that, we make sure that the test list is up-to-date, and
that added tests are included in the next run.
I've also removed the `listChanged` event as discussed in the team sync.
The event isn't used anywhere and fires in exactly the same cases where
`testFilesChanged` fired, so i've folded them into one another. This allowed simplifying `Watcher`.
