Add more detailed WPT test coverage for stopping behavior.

The added tests aim to cover:
- When transceiver.recevier.track should or should not end.
- When transceiver is stopped locally versus remotely.
- When the transceiver is "stopping" or "stopped".
- When transceiver is stopped due to being removed in a rollback versus
  being rolled back without being removed.

The -expected.txt file reflects https://crbug.com/1315611 being fixed,
but the following issues have not been fixed yet which FAIL:
- https://crbug.com/1319913: direction is not updated to "stopped" if
  transceiver is stopped remotely.
- https://crbug.com/1319911: currentDirection is not updated to
  "stopped" when already "stopping" and answer removes the transceiver.

// Test coverage for the following issues:

Bug: 1315611, 1319902, 1319911, 1319913, 1319924
Change-Id: I8ad97221f5d2171384e8c7be32eaabd9811ba8f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3609997
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#997067}