RFR: 8337111: Bad HTML checker for generated documentation [v4]

[Nizar Benalla]

> Can I please get a review for this PR that adds 4 new html "Checkers" for the generated documentation.
> More details are in the JBS issues
> 
> These tests were mostly inspired /converted from the existing [Doccheck](https://github.com/openjdk/doccheck).

Nizar Benalla has updated the pull request incrementally with one additional commit since the last revision:

  make test run with executors by default

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20711/files
  - new: https://git.openjdk.org/jdk/pull/20711/files/c6d60f95..19d07811

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20711&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20711&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20711.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20711/head:pull/20711

PR: https://git.openjdk.org/jdk/pull/20711