[dune_add_test] Do not check existance of EXPECT_COMPILE_FAIL targets.

Fixes: #100.  See also: !404.

Those targets never exist -- instead, cmake is invoked during testing to
attempt to build them.

cmake/modules/DuneTestMacros.cmake

@@ -303,7 +303,9 @@ function(dune_add_test)
                )
 
       # Make the test depend on the existence of the target to trigger "Not Run" response
-      set_tests_properties(${ACTUAL_NAME} PROPERTIES REQUIRED_FILES ${ADDTEST_TARGET})
+      if(NOT ADDTEST_EXPECT_COMPILE_FAIL)
+        set_tests_properties(${ACTUAL_NAME} PROPERTIES REQUIRED_FILES ${ADDTEST_TARGET})
+      endif()
       # Define the number of processors (ctest will coordinate this with the -j option)
       set_tests_properties(${ACTUAL_NAME} PROPERTIES PROCESSORS ${procnum})
       # Apply the timeout (which was defaulted to 5 minutes if not specified)