For example, in the following abbreviated test HTML:
<span>some text</span>
<script>println("whf")</script>
We would have to craft the expectation file to include the "some text"
segment, usually with some leading whitespace. This is a bit annoying,
and makes it difficult to manually craft expectation files.
So instead of comparing the expectation against the entire DOM inner
text, we now send the inner text of just the <pre> element containing
the test output when we invoke `internals.signalTextTestIsDone`.
This piggybacks on the same fragment serialization code that innerHTML
uses, but instead of constructing an imaginary parent element like the
spec asks us to, we just add a separate serialization mode that includes
the context element in the serialized markup.
This makes the image carousel on https://utah.edu/ show up :^)
