Add test for gather_power_measurement_results.py

So we don't get surprises when we try to run it, it doesn't work because
buildbucket APIs changed or step stdout format changed.

BUG=867155
TEST=bots
R=kbr@chromium.org,dpranke@chromium.org

Change-Id: Iaed55b1f262f1ccd3b52b015feae41b08ad2abc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1576161
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652943}
3 files changed