[LCP WPT] Fix test expectation in background-image-set-image.html

[chromium-wpt-export-bot]

The step_timeout usage in the original version of the test made it so that it wouldn't actually check the assertion before exiting successfully.

Fixing this by only resolving the promise if we found lcp-100x50.

Change-Id: If4a4e485a1ef3b915bb7d2616f20e9dc8ab47554 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6641890 Reviewed-by: Scott Haseley <[email protected]> Commit-Queue: Johannes Henkel <[email protected]> Cr-Commit-Position: refs/heads/main@{#1473835}