Issue #21741: Update 147 test modules to use test discovery.

I have compared output between pre- and post-patch runs of these tests
to make sure there's nothing missing and nothing broken, on both
Windows and Linux.  The only differences I found were actually tests
that were previously *not* run.
diff --git a/Lib/test/test_userdict.py b/Lib/test/test_userdict.py
index 2ca9929..c83cc55 100644
--- a/Lib/test/test_userdict.py
+++ b/Lib/test/test_userdict.py
@@ -190,10 +190,5 @@
 
 
 
-def test_main():
-    support.run_unittest(
-        UserDictTest,
-    )
-
 if __name__ == "__main__":
-    test_main()
+    unittest.main()