Refactor gpu_integration_test_unittest.py by removing mock classes

This CL will replace the mock classes defined in
gpu_integration_test_unittest.py with telemetry's mock classes
defined in it's testing.fakes subpackage.

Bug: chromium:976505
Change-Id: I256e9273d042f121c0dbdeb74bc6c895bcdea27a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666463
Commit-Queue: Rakib Hasan <rmhasan@google.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670677}