Fix tests so they pass in -R mode
diff --git a/Lib/test/test_difflib.py b/Lib/test/test_difflib.py
index 52feef0..83fad15 100644
--- a/Lib/test/test_difflib.py
+++ b/Lib/test/test_difflib.py
@@ -152,6 +152,10 @@
         difflib.SequenceMatcher(None, old, new).get_opcodes()
 
 
-Doctests = doctest.DocTestSuite(difflib)
+def test_main():
+    difflib.HtmlDiff._default_prefix = 0
+    Doctests = doctest.DocTestSuite(difflib)
+    run_unittest(TestSFpatches, TestSFbugs, Doctests)
 
-run_unittest(TestSFpatches, TestSFbugs, Doctests)
+if __name__ == '__main__':
+    test_main()