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`.
We now follow the rules from the spec more closely, along with an
unspecified quirk for when the offsetParent is a non-positioned body
element. (Spec bug linked in a comment.)
This fixes a whole bunch of css-flexbox tests on WPT, which already had
correct layout, but the reported metrics from JS API were wrong.
