Add a new `assertEqualHTML` method to `WP_UnitTestClass` for tests comparing HTML (potentially including block markup).
Internally, the assertion builds a deterministic tree string representation of the markup (using the HTML API) and compares the results. The format of the tree is inspired by the HTML5lib-tests tree format. It is extended with a special representation of block delimiters and their attributes. This format also makes it easier to visually spot the differences between the two strings if the assertion fails.
Finally, this changeset updates `Tests_Dependencies_Scripts` to remove its `assertEqualMarkup` and `parse_markup_fragment` methods, and to use the newly introduced `assertEqualHTML` instead.
Props bernhard-reiter, jonsurrell, dmsnell, jorbin, gziolo.
Fixes#63527.
git-svn-id: https://develop.svn.wordpress.org/trunk@60295 602fd350-edb4-49c9-b593-d223f7449a82
