[CMake] Add a test target for test/.

All folders called test/ are automatically added to CTest
by Dune and should provide a test target. Currently it is
a bug that a directory directly below the module is
ignored. Adding the target makes us future-proof.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12769 2fb0f335-1f38-0410-981e-8018bf24f1b0