Allow cpp error test.

It's really sad that we don't have any of these yet.

runtests.py

@@ -178,6 +178,9 @@ class TestBuilder(object):
 
     def build_tests(self, test_class, path, workdir, module, expect_errors):
         if expect_errors:
+            if 'cpp' in module and 'cpp' in self.languages:
+                languages = ['cpp']
+            else:
                 languages = self.languages[:1]
         else:
             languages = self.languages