      def test_check_compatibility(self):
          with self.settings(TEST_RUNNER='django.test.runner.DiscoverRunner'):
              result = base.check_compatibility()
              self.assertEqual(len(result), 1)
              self.assertTrue("You have not explicitly set 'TEST_RUNNER'" in result[0])
  
          with self.settings(TEST_RUNNER='myapp.test.CustomRunnner'):
              self.assertEqual(len(base.check_compatibility()), 0)
  
      def test_check_compatibility_warning(self):
 
</s> remove             self.assertTrue("You have not explicitly set 'TEST_RUNNER'" in result[0])
 </s> add             self.assertTrue("Django 1.6 introduced a new default test runner" in result[0]) </s> remove             self.assertTrue("You have not explicitly set 'TEST_RUNNER'" in result)
 </s> add             self.assertTrue("Django 1.6 introduced a new default test runner" in result) </s> remove         self.assertTrue("You have not explicitly set 'TEST_RUNNER'" in check.warnings._warnings[0])
 </s> add         self.assertTrue("Django 1.6 introduced a new default test runner" in check.warnings._warnings[0]) </s> remove             "You have not explicitly set 'TEST_RUNNER'. In Django 1.6,",
            "there is a new test runner ('%s')" % new_default,
            "by default. You should ensure your tests are still all",
            "running & behaving as expected. See",
 </s> add             "Django 1.6 introduced a new default test runner ('%s')" % new_default,
            "You should ensure your tests are all running & behaving as expected. See",
            self.assertTrue("Django 1.6 introduced a new default test runner" in result[0])
      def test_check_compatibility(self):
          with self.settings(TEST_RUNNER='django.test.runner.DiscoverRunner'):
              result = base.check_compatibility()
              self.assertEqual(len(result), 1)
            self.assertTrue("Django 1.6 introduced a new default test runner" in result[0])
  
          with self.settings(TEST_RUNNER='myapp.test.CustomRunnner'):
              self.assertEqual(len(base.check_compatibility()), 0)
  
      def test_check_compatibility_warning(self):
keep keep keep keep replace keep keep keep keep keep
 Refs #21197 -- Clarified upgrade check message.

Thanks to Carl and Shai for the discussion. 
tests/check/tests.py