Test Runner: Fix dirty thread teardown. When the standalone tests shut down they would not wait for the watchdog thread to exit cleanly. Fix this by setting a flag that shuts down the watchdog and attempting to wait. This was detected by running angle_end2end_tests with TSAN. Bug: angleproject:3162 Bug: b/168744561 Change-Id: I765ce7a21c3be11703619470a81adb1882b009e4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2415175 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>