We added run_gtest_angle_test.py in a86c8547, but didn't
actually check that it worked :(. This CL adds the
missing sys.path imports needed to find the rest of
the Chromium test script modules.

Bug: chromium:816629, angleproject:3162
Change-Id: I4c381bda334df4d2e648aab65ca4cc0310d840bf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2283694Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>