Thanks to @Zegnat and @sebsel for finding some new new edges case in Webmention discovery that deserve new tests!
The new test, #22, advertises its Webmention endpoint with a URL that is relative to the page (e.g.
<link rel="webmention" href="22/webmention">). The existing relative URL tests were absolute paths (e.g.
<link rel="webmention" href="/test/22/webmention">).
If you already handle relative URL resolution with a library then chances are you will pass this test without any new code.