Tests: Extend CMakeOnly.CheckLanguage to enable languages whose checks succeed

Tests/CMakeOnly/CheckLanguage/CMakeLists.txt

@@ -30,4 +30,7 @@ foreach(test_lang ${LANGUAGES})
       message(FATAL_ERROR "check_language(${test_lang}) should not succeed!")
     endif()
   endif()
+  if(CMAKE_${test_lang}_COMPILER)
+    enable_language(${test_lang})
+  endif()
 endforeach()