LibWeb+WebContent: Do not include DOM HTML in text test expectations

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`.

(cherry picked from commit bf668696de9a68bbcd6adfeaac809a475da015dc;
amended a bit but not as much as one might think -- see PR for details)

Tests/LibWeb/Text/expected/HTML/HTMLAllCollection-basic.txt

@@ -1,4 +1,4 @@
-      HTMLAllCollection
+HTMLAllCollection
 typeof document.all = undefined
 equal to undefined = true
 equal to null = true