Adding code in ANGLETestEnvironment::TearDown() to release fixtures if exists.

Bug: angleproject:3875
Change-Id: I0d6ad9b8c72937b20a4c3033b86eb667b546e0c3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1778841Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Rafael Cintron <rafael.cintron@microsoft.com>