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_memoryio.py

@@ -826,10 +826,5 @@ class CStringIOPickleTest(PyStringIOPickleTest):
             pass
 
 
-def test_main():
-    tests = [PyBytesIOTest, PyStringIOTest, CBytesIOTest, CStringIOTest,
-             PyStringIOPickleTest, CStringIOPickleTest]
-    support.run_unittest(*tests)
-
 if __name__ == '__main__':
-    test_main()
+    unittest.main()