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_dynamicclassattribute.py

@@ -4,7 +4,6 @@
 import abc
 import sys
 import unittest
-from test.support import run_unittest
 from types import DynamicClassAttribute
 
 class PropertyBase(Exception):
@@ -297,8 +296,5 @@ class PropertySubclassTests(unittest.TestCase):
 
 
 
-def test_main():
-    run_unittest(PropertyTests, PropertySubclassTests)
-
 if __name__ == '__main__':
-    test_main()
+    unittest.main()