Combine the functionality of test_support.run_unittest()
and test_support.run_classtests() into run_unittest()
and use it wherever possible.

Also don't use "from test.test_support import ...", but
"from test import test_support" in a few spots.

From SF patch #662807.
diff --git a/Lib/test/test_parser.py b/Lib/test/test_parser.py
index ccdfd19..7860e7b 100644
--- a/Lib/test/test_parser.py
+++ b/Lib/test/test_parser.py
@@ -374,11 +374,10 @@
         self.check_bad_tree(tree, "malformed global ast")
 
 def test_main():
-    loader = unittest.TestLoader()
-    suite = unittest.TestSuite()
-    suite.addTest(loader.loadTestsFromTestCase(RoundtripLegalSyntaxTestCase))
-    suite.addTest(loader.loadTestsFromTestCase(IllegalSyntaxTestCase))
-    test_support.run_suite(suite)
+    test_support.run_unittest(
+        RoundtripLegalSyntaxTestCase,
+        IllegalSyntaxTestCase
+    )
 
 
 if __name__ == "__main__":