report_stages: Use the full GSContextMock.

If a test tries to mock a specific GS function, but doesn't use
GSContextMock, it can block in run_tests while trying (inside a retry
loop) to fetch gsutil.

This can block other tests who also try to fetch gsutil.

BUG=chromium:4822235
TEST=Ran tests in conditions that reproduce the bug.

Change-Id: Ibcf7fe078c979cc189cde3de3912c0ca4fdcc329
Reviewed-on: https://chromium-review.googlesource.com/267786
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
1 file changed