doc: Enable doxygen on the test directory #19248

While it makes sense to exclude third party libraries in subtrees from doxygen, is there any reason to also exclude the fuzz and unit tests?

They both have frameworks and utility functions that I need to look up repeatedly.

Doxygen is meant to be developer documentation and not Bitcoin Core user documentation, so I see no downside of enabling the test directory in doxygen. Am I missing something?

Concept ACK, I don't remember anymore why I disabled the test, but whatever reason I had is likely not valid anymore.

Edit: the reason was to avoid clutter, doxygen, at least at the time would be confused by the many entry points and things of all the different tests.