[cmake] ensure test binaries are build with dune's own make test

cmake/modules/DuneUtils.cmake

@@ -125,6 +125,8 @@ macro(BEGIN_TESTCASES)
 endmacro(BEGIN_TESTCASES)
 
 macro(END_TESTCASES)
+    add_directory_test_target(_test_target)
+    add_dependencies(${_test_target} ${testnames})
 	add_custom_target(test_binaries DEPENDS ${testnames})
 	add_custom_target(check COMMAND ${CMAKE_CTEST_COMMAND}
                   DEPENDS test_binaries)