This adds printing on failures to compare output when correctness fuzzing.

TBR=yangguo@chromium.org
NOTRY=true

Bug: chromium:813833
Change-Id: I0280561392f6614960608db4e93341552c716142
Reviewed-on: https://chromium-review.googlesource.com/934843
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51529}