fix: disable DD4Hep tests in cmake

Came across this during reference bot developments.

Returning exit(77) from a test kind of works, but not perfectly. While the test registers as "skipped", validation is still executed and this results in newly created empty reference files. This would then result in automatic reference updates erasing the ref files of DD4Hep tests.

In this case, it is known at configuration time if those tests should be run or not, so I've moved the skipping to the CMake level.