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 can't rely on Element.setAttribute() in cloneNode() since that will
throw on weird attribute names. Instead, just follow the spec and copy
attributes into cloned elements verbatim.
This fixes a crash when loading the "issues" tab on GitHub repos.
They are actually sending us unintentionally broken markup, but we
should still support cloning it. :^)
