Add expected string to async_test

This patch supplies a second param to the async_test in
destroy-middle-click-locked-target-crash.html ensuring that the
test always produces the same output when it passes. The linked
bug shows test flakes that occurred because the test sometimes
creates output (and sometimes does not).

Bug: 1273003
Change-Id: I5d87907080fd0107f8993cdfc84144b33f4a3be4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3946598
Quick-Run: David Awogbemila <awogbemila@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Auto-Submit: David Awogbemila <awogbemila@chromium.org>
Commit-Queue: David Awogbemila <awogbemila@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1057677}