HTML API: WP_HTML_Processor::next_token nests void tags

[jonsurrell]

When processing HTML like <br><br> with WP_HTML_Processor::next_tag(), 2 sibling BR tags are correctly found.

However, when using WP_HTML_Processor::next_token(), void tags are not correctly handled, resulting in breadcrumbs from the second BR tag like [ 'HTML', 'BODY', 'BR', 'BR' ].

• Use $p variable for processor like other tests
• Add failing test

Trac ticket: https://core.trac.wordpress.org/ticket/60382

[jonsurrell]

This was found thanks to the external test suite proposed in #60227.

[jonsurrell]

HTML API: WP_HTML_Processor::next_token is an internal API (with @access private) so this may not be considered a bug.

Trac ticket: Core-60382

This currently only includes a failing test.

@dmsnell your fix looks good to me and fixes the test failures I saw in ​https://github.com/WordPress/wordpress-develop/pull/5794.

👍 This change looks good to me.

[dmsnell]

In 57507:

HTML API: Fix void tag nesting with next_token

When next_token() was introduced, it introduced a regression in the HTML
Processor whereby void tags remain on the stack of open elements when they
shouldn't. This led to invalid values returned from get_breadcrumbs().

The reason was that calling next_token() works through a different code path
than the HTML Processor runs everything else. To solve this, its sub-classed
next_token() called step( self::REPROCESS_CURRENT_TOKEN ) so that the proper
HTML accounting takes place.

Unfortunately that same reprocessing code path skipped the step whereby void
and self-closing elements are popped from the stack of open elements.

In this patch, that step is run with a third mode for step(), which is the
new self::PROCESS_CURRENT_TOKEN. This mode acts as if self::PROCESS_NEXT_NODE
were called, except it doesn't advance the parser.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/5975
Discussed in https://core.trac.wordpress.org/ticket/60382

Follow-up to [57348]

Props dmsnell, jonsurrell
Fixes #60382

Merged in [57507]
​https://github.com/wordpress/wordpress-develop/commit/cdb218b2007693262f2ea2d9717ec0ff55620b80