tests: Add htmldocck.py script for the use of Rustdoc tests.

The script is intended as a tool for doing every sort of verifications
amenable to Rustdoc's HTML output. For example, link checkers would go
to this script. It already parses HTML into a document tree form (with
a slight caveat), so future tests can make use of it.

As an example, relevant `rustdoc-*` run-make tests have been updated
to use `htmldocck.py` and got their `verify.sh` removed. In the future
they may go to a dedicated directory with htmldocck running by default.
The detailed explanation of test scripts is provided as a docstring of
htmldocck.

cc #19723

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