HTML API: Validate HTML Processor against external test suite from html5lib.

In this patch, the test suite from html5lib validates the tree-construction
steps in the HTML Processor to ensure that they are behaving according to the
HTML specification. This suite of tests is also used by the servo project to
test its html5ever package.

A new test module in the HTML API transforms HTML Processor output to match
the expected tree shape from the external tests. For cases where there are
tests validating behaviors of unsupported HTML tags and constructs, the tests
are marked as skipped. As the HTML API continues to expand its own support,
the number of skipped tests will automatically shrink down towards zero.

Additional tests are skipped through the `SKIP_TEST` array in the test runner.

Fixes #60227.
See #58517.
Props azaozz, costdev, dmsnell, hellofromtonya, jonsurrell, jorbin, swisspidy.


Built from https://develop.svn.wordpress.org/trunk@58010


git-svn-id: http://core.svn.wordpress.org/trunk@57481 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.6-alpha-58009';
+$wp_version = '6.6-alpha-58010';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.