runtime(html): Fix syntax test failures

[dkearns]

Problem: htmlItalic highlight group dump file differences on macOS are
causing test failures.
Solution: Remove content from <em> and <i> element examples in the HTML
test file.

The included content for these elements was incidental to the primary
intent of the test to check element tag highlighting. See #13591.

[dkearns]

It seems that even an empty <strike></strike> element breaks the HTML syntax test on FreeBSD but not macOS.

testdir/failed/html_08.dump is the reported failure. I can't see a specific dump file line reported but commenting out the strike element, which also uses a custom strikethrough highlight group, results in a passing test.

I don't have any experience with screen dumps. Are they usually reliable across platforms?

[chrisbra]

I don't know, but perhaps the FreeBSD CI does not support the strikethrough attribute? I don't think we have tested that one before in a screen-dump test. Perhaps we should leave that one out (or skip the test on BSD and Mac) in this case?