[3.14] gh-135661: Fix abrupt closing of empty comment in HTMLParser (GH-153007) ...

[3.14] gh-135661: Fix abrupt closing of empty comment in HTMLParser (GH-153007) (GH-153025)

An abruptly closed empty comment ("<!-->" or "<!--->") no longer extends
up to a later "-->" in the same feed() call.

test_htmlparser now also feeds each string source as a single chunk, in
addition to one character at a time, to exercise different input buffering.
(cherry picked from commit ed370d3337c1ac4f7295b444a4807c0b72287608)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>