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>
