A number of folks use a single page for multiple posts, using a URL
Fragment parameter to denote which of the posts on the given page should
be used, and are used to this experience with Bridgy.
To allow use of this on Bridgy Fed, too, we can add support for
discovering the ID of the `h-entry` to be Webmention'd in the same way
that we do with [Bridgy].
This largely copies the structure of the existing tests, copying
`test_activitypub_follow`, and adding in multiple posts on the page.
For debugging purposes, we can make sure to log out the `fragment`.
[Bridgy]: f760c0d10e