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.

Lib/test/test_stringprep.py

@@ -2,7 +2,6 @@
 # Since we don't have them, this test checks only a few code points.
 
 import unittest
-from test import support
 
 from stringprep import *
 
@@ -89,8 +88,5 @@ class StringprepTests(unittest.TestCase):
         #     h.update(data)
         #     print p, h.hexdigest()
 
-def test_main():
-    support.run_unittest(StringprepTests)
-
 if __name__ == '__main__':
-    test_main()
+    unittest.main()