GTEST_SKIP() allows us to skip tests using the GoogleTest APIs.
This CL updates our test runner code to handle these skips.

Bug: angleproject:5951
Change-Id: Idef5802c7c81bd92226f5a93281a1cea0f1b24ee
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2892272Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>