I would run that test with verbosity (ctest -R '$testname' -VV) to see how CTest is invoking it. Things to note:
CTest may be setting extra environment settings (via the ENVIRONMENT property)
Clang could have a bug (6 is pretty old these days); release-only points to something optimizer-related
For debugging, I would run the test inside and outside of CTest using valgrind to see if it finds anything (you’ll need to modify the add_test to have it run under CTest). You can also try the sanitizers (ASan and UBSan are of particular note), though Clang 6’s sanitizers may be less useful than new ones (that said, new ones could also point the problem out without crashing too).