When one comparison run crashes, we cap the outputs to compare to the
shorter one. If one of those, however, contains ignored lines, the
comparison get's skewed.

This makes the main source of ignored lines more robust (the line
printed for unknown flags), by not printing it in the first place in
the context of differential fuzzing.

Bug: chromium:1153871
Change-Id: If2e534959779be14a686be5e43630cbf66e215a0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567692
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71522}