Test Runner: Use system path for gtest-internal. This ensures we use the correct internal path in Chromium builds as well. Bug: angleproject:3162 Change-Id: I036dcbd8802f67162e5f5eccb6158910f0a5efcc Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2082134 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>