Add Java 16 Linux matcher variant to HtmlDecorator

Add Java 16 Linux matcher variant to HtmlDecorator

The Java 16 Javadoc generator has changed the HTML structure once again
even compared to Java 15. I adjusted the matching in HtmlDecorator and
also fixed CoverageTestCase. Most methods there I just made to work
quickly, but method 'testInnerAspect()' I actually refactored. Some
other methods could (probably should) be restructured in a similar
fashion, but for now I just wanted to understand what the test does and
see how much work it would be to refactor it. But finally, I just want
to get the GitHub CI build running on Java 16.

TODO: I did not check if the decorated HTML actually looks OK and am
unsure if the tests cover that sufficiently, I never reviewed the tests.
It would also be better to do regex matches instead of looking for
variants of fixed strings or maybe even to operate on a DOM. But I am
not in a mood to refactor that tonight.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>