Port CppUnit tests to Catch2 and improve Catch2 support

After having to update a CppUnit test in !1826 (merged) I decided to drop CppUnit support and migrate (with help from an AI agent) the few CppUnit tests we had to Catch2.

This allowed me to discover that long time ago we introduced a duplicated test executable that I now removed.

Along with the conversion of the tests, I dropped CppUnit support and dependency and addressed #377 (closed) (at least for the Catch2 case).

Closes #377 (closed)