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_fractions.py b/Lib/test/test_fractions.py
index e86d5ce..2dd528f 100644
--- a/Lib/test/test_fractions.py
+++ b/Lib/test/test_fractions.py
@@ -1,7 +1,7 @@
 """Tests for Lib/fractions.py."""
 
 from decimal import Decimal
-from test.support import run_unittest, requires_IEEE_754
+from test.support import requires_IEEE_754
 import math
 import numbers
 import operator
@@ -606,8 +606,5 @@
         r = F(13, 7)
         self.assertRaises(AttributeError, setattr, r, 'a', 10)
 
-def test_main():
-    run_unittest(FractionTest, GcdTest)
-
 if __name__ == '__main__':
-    test_main()
+    unittest.main()