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`.
When an <input type=image> button is clicked, we now send the (x,y)
coordinates of the click event (relative to the image) along with the
form submission data.
Regarding the text test, we can currently only test this feature with
dialogs. The headless-browser test infrastructure cannot yet handle the
resulting navigation that would occur if we were to test with normal
form submission.
