run-tests: produce '!' mark after running a failing test

This patch  fixes a regression recently introduced by a refactoring (see
92a6b16c9186 and about 200 previous changesets from Gregory Szorc).  '!' mark
denotes that the test gets failed while testing.

tests/run-tests.py

@@ -1079,6 +1079,7 b' class TestResult(unittest._TextTestResul'
         else:
             if not self._options.nodiff:
                 self.stream.write('\nERROR: %s output changed\n' % test)
+            self.stream.write('!')
 
     def addError(self, *args, **kwargs):
         super(TestResult, self).addError(*args, **kwargs)