Tests: Make the beforeunload event tests work regardless of extensions

[mgol]

Summary

Some browser extensions, like React DevTools, send messages to the content area. Since our beforeunload event test listens for all messages, it used to catch those as well, failing the test.

Add a source field to the payload JSON and check for it before treating the message as coming from our own test to make sure the test passes even with such browser extensions installed.

Checklist

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

[mgol]

Landed on main in 399a78ee9fc5802509df462a2851aef1b60b7fbc and on 3.x-stable in 5d83fadb6be83bcda54a87c1c0d73534e5aad6fb.