Test Runner: Accept GTEST shard environment vars.
Test Runner: Accept GTEST shard environment vars.

This makes interoperation with Chromium tests easier. For instance when
triggering tests with scripts when using sharding. And also using the
run/ wrapper scripts instead of run_gtest_angle_test.

Bug: angleproject:5158
Change-Id: Ia5b1f94c02a081caf576615175791fff4e0d3fb3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2464030
Reviewed-by: Courtney Goeltzenleuchter <aa371ec5c940a0ded89f7ed95bce84faebcf51df@google.com>
Reviewed-by: Yuly Novikov <36f5580f63d0a3eacc17e89a61d11e2f646630d2@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle