[3.6] Improve test suite customization example (GH-3967)

Reported by John Gamboa on docs@p.o at
https://mail.python.org/pipermail/docs/2017-June/031942.html

(cherry picked from commit 92551044)

Doc/library/unittest.rst

@@ -402,10 +402,14 @@ you can do it yourself::
 
    def suite():
        suite = unittest.TestSuite()
-       suite.addTest(WidgetTestCase('test_default_size'))
-       suite.addTest(WidgetTestCase('test_resize'))
+       suite.addTest(WidgetTestCase('test_default_widget_size'))
+       suite.addTest(WidgetTestCase('test_widget_resize'))
        return suite
 
+   if __name__ == '__main__':
+       runner = unittest.TextTestRunner()
+       runner.run(suite())
+
 You can place the definitions of test cases and test suites in the same modules
 as the code they are to test (such as :file:`widget.py`), but there are several
 advantages to placing the test code in a separate module, such as