[gold] Clean up cmd_imgtest
[gold] Clean up cmd_imgtest

Change-Id: I4fe9123fa7bd8cf5cdb2c78c571c468e95d7394c
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/243641
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Reviewed-by: Ben Wagner aka dogben <3ef7217be91069877d94f7907ce5479000772cd3@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot