[CMake] Build test in make dependant if according parameter is set.

Tests are still built only for ctest calls, not for plain make.
Tests are built for make if -DDUMUX_BUILD_ALL_TESTS:BOOL=TRUE is set.
This change is needed for CDash, otherwise compile errors and warnings
where not displayed.

(reviewed by bernd)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11537 2fb0f335-1f38-0410-981e-8018bf24f1b0