Clearing the GTEST args from the env vars will run the proper subset
of tests instead of applying sharding arguments twice. This should
fix the tests on the bots running a small subset of the tests.

Also updates logging control for the script.

Bug: angleproject:5966
Change-Id: Ic8a21e6aacfed17eff836d4301a6a50464b2cb92
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2935196Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>