Fix dm/viewer startup to avoid loading images unnecessarily.

We should only load images if the test is actually executed.

Change-Id: I1a84354831259c8d9f8010147ea15f1beae59128
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/770597
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: James Godfrey-Kittle <jamesgk@google.com>
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
3 files changed