The bot that runs gcmole was failing before
https://crrev.com/c/1789707 because the test file was missing.
It returned with exit status 0 anyway though. After fixing the
original fault, this CL ensures that the gcmole tests also
trigger an error on the bot(s) if they fail.

R=mstarzinger@chromium.org
CC=​​mslekova@chromium.org

Change-Id: I29ae40301062baadfcd38b26c336c5749924b0d4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1789702
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63612}