Tests: fix flakey message logs; ignore delete worker failures

[timmywil]

Summary

Add some minor improvements after watching the recent first run on 3.x-stable, including:

• fix the flakey message for when a test fails and then passes on retry.
• ignore worker delete failures. We hit this in one browser. The tests all passed and the worker had stopped, but BrowserStack returned a 502.

Checklist

• [x] New tests have been added to show the fix or feature works
• [x] If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com