When a user specifies a test argument which does not exist, run-tests doesn't explain to the user what a valid argument looks like.
<rdar://problem/111348949>
Pull request: https://github.com/WebKit/WebKit/pull/15305
Committed 266108@main (ce2b6a5f42c5): <https://commits.webkit.org/266108@main> Reviewed commits have been landed. Closing PR #15305 and removing active labels.