Fix test failures against Drupal 10.2

The attached patch should fix the tests. (Not really possible to test with the bot, unfortunately.)

The fix for HtmlFilter is, of course, less than ideal – since the new version of the method was the problem, I simply copied the old one. I felt a much more elegant way would have been to avoid serializing the DOM object again and instead iterating through its contents, building our text tokens from that. However, iterating over the contents of a \DOMDocument object is apparently not that simple, so I opted for this solution instead. Other suggestions very much welcome!

Merged.

Automatically closed - issue fixed for 2 weeks with no activity.