run-tests: produce error on running a failing test

This patch fixes a regression recently introduced by a refactoring (see
92a6b16c9186 and about 200 previous changesets from Gregory Szorc).  It produce
an error message everytime with a test filename which gets fail while testing
except at one condition when '--nodiff' option is enabled.

tests/run-tests.py

@@ -1076,6 +1076,9 b' class TestResult(unittest._TextTestResul'
 
         if self._options.first:
             self.stop()
+        else:
+            if not self._options.nodiff:
+                self.stream.write('\nERROR: %s output changed\n' % test)
 
     def addError(self, *args, **kwargs):
         super(TestResult, self).addError(*args, **kwargs)