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`.
If initial src of an iframe is "about:blank", it does synchronous
navigation that is not supposed to be interleaved by other navigation
or usage of Document.open().
Fixes crashing in navigation on https://twinings.co.uk/
