HTML API: HTML Processor bookmarks seek, has_bookmark bugs

[jonsurrell]

WP_HTML_Processor::has_bookmark does not correctly report bookmarks that have been set.

WP_HTML_Processor::seek does not seek to correct location if HTML has been updated.

• Add test suite for WP_HTML_Processor bookmark functions.
• Fix issue with ::seek when HTML has been modified.
• Fix issue with ::has_bookmark not reporting correctly for set bookmarks.

Trac ticket: https://core.trac.wordpress.org/ticket/60474

[dmsnell]

In 57582:

HTML API: Add subclassed has_bookmark() and fix seek()

The WP_HTML_Processor::has_bookmark() method has not correctly reported bookmarks
which have been set, because it wraps the given bookmark names when setting them.
Additionally, WP_HTML_Processor::seek() does not seek to correct location if HTML
has been updated because it wasn't flushing enqueued updates to the document.

In this patch both problems are resolved and added tests guard these behaviors
against future regressions.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/6039
Discussed in https://core.trac.wordpress.org/ticket/60474

Follow-up to [56274].
Props dmsnell, jonsurrell.
Fixes #60474.

Merged in [57582]
​https://github.com/wordpress/wordpress-develop/commit/44a2073816ba8c474f598c348e4d334696b495e4