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`.
Change `EventHandler::handle_keydown()` to no longer assume the cursor
position's node is always a `DOM::Text`. While this assumption holds
for `HTMLInputElement` that has a shadow DOM with a text node, an empty
`contenteditable` might not have any children. With this change,
`handle_keydown()` creates a new text node if the cursor position's
node is not a text node.
