Flaky failures were being hidden by test retries. Should be ignored so
we can disable test retries.

Bug: angleproject:3271
Change-Id: I17708f96e2299b9828f5f979f68187b3cd99ecb3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1542259Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>