Update help_topic tests to HTML5

@smustgrave, thanks for creating the follow-up issue :+1:

I have started by pushing the latest state of the changes I introduced on the help topic specific test from the original issue, to the html5-tests-help-topics branch.
I also pushed the work-around so it is clear what is failing, to html5-tests-help-topics-with-workaround branch.
Both have related MRs for the test run.

Marking as NW, next step is trying to fix the failing format check edge case.

Updated summary a bit - ref to discussion where split proposed

Maybe it will be easier to add another bad_html_v5 topic to have both expectations as both xhtml and html5 remains maintained in future version of PHP

btw HTML5 compatible classes just commited to PHP 8.4 https://wiki.php.net/rfc/opt_in_dom_spec_compliance

in a light of PHP 8.4 https://wiki.php.net/rfc/opt_in_dom_spec_compliance

Core can check PHP version and use faster validator https://3v4l.org/IhHj3/rfc#vgit.master

$ php -r 'var_dump((DOM\HTMLDocument::createEmpty())->relaxNgValidateSource("<p><a href=\"/foo\">Text here</p>"));'
PHP Warning:  Dom\Document::relaxNgValidateSource(): Entity: line 1: parser error : Opening and ending tag mismatch: a line 1 and p in Command line code on line 1

Warning: Dom\Document::relaxNgValidateSource(): Entity: line 1: parser error : Opening and ending tag mismatch: a line 1 and p in Command line code on line 1
PHP Warning:  Dom\Document::relaxNgValidateSource(): <p><a href="/foo">Text here</p> in Command line code on line 1

Warning: Dom\Document::relaxNgValidateSource(): <p><a href="/foo">Text here</p> in Command line code on line 1
PHP Warning:  Dom\Document::relaxNgValidateSource():                                ^ in Command line code on line 1

Warning: Dom\Document::relaxNgValidateSource():                                ^ in Command line code on line 1
PHP Warning:  Dom\Document::relaxNgValidateSource(): xmlRelaxNGParse: could not parse schemas in Command line code on line 1

Warning: Dom\Document::relaxNgValidateSource(): xmlRelaxNGParse: could not parse schemas in Command line code on line 1
PHP Warning:  Dom\Document::relaxNgValidateSource(): Invalid RelaxNG in Command line code on line 1

Warning: Dom\Document::relaxNgValidateSource(): Invalid RelaxNG in Command line code on line 1
bool(false)