Avoid unneccessary work if tests are not requested
Some tests are built (and installed, see #244 (closed)) even if BUILD_TESTING is set to false.
This MR fixes the problem (note that gaudi_add_tests() already respects BUILD_TESTING).
Some tests are built (and installed, see #244 (closed)) even if BUILD_TESTING is set to false.
This MR fixes the problem (note that gaudi_add_tests() already respects BUILD_TESTING).