Issue #3090257 by jhodgdon, andypost: Add additional tests for help topic syntax

core/modules/help_topics/help_topics/book.about.html.twig

...les/help_topics/help_topics/contextual.overview.html.twig

...dules/help_topics/help_topics/core.web_services.html.twig

core/modules/help_topics/help_topics/forum.concept.html.twig

...odules/help_topics/help_topics/migrate.overview.html.twig

...modules/help_topics/help_topics/search.overview.html.twig

...ad_help_topics/syntax/bad_help_topics.bad_html2.html.twig

...ad_help_topics/syntax/bad_help_topics.bad_html3.html.twig

...help_topics/tests/src/Functional/HelpTopicsSyntaxTest.php