[CMake] Adjust macro add_dumux_test to not violate CMP00046.

With CMake 3.0 warns about dependencies to non-existent targets.
(reviewed and tested by bernd)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12619 2fb0f335-1f38-0410-981e-8018bf24f1b0