Directories containing tests need an additional 'make' target

Otherwise you get warnings complaining about a missing file 'BuildTest.cmake'.

dune/tnnmg/nonlinearities/test/CMakeLists.txt

@@ -3,6 +3,9 @@ set(TESTS
   testminimalsurfacefunctional
 )
 
+add_directory_test_target(_test_target)
+add_dependencies(${_test_target} ${TESTS})
+
 foreach(_test ${TESTS})
   add_executable(${_test} ${_test}.cc)
   target_link_libraries(${_test} ${DUNE_LIBS})