Fix so that test.test_unittest can be executed by unittest and not just regrtest

Lib/test/test_unittest.py

@@ -4,8 +4,13 @@ from test import support
 
 
 def test_main():
+    # used by regrtest
     support.run_unittest(unittest.test.suite())
     support.reap_children()
 
+def load_tests(*_):
+    # used by unittest
+    return unittest.test.suite()
+
 if __name__ == "__main__":
     test_main()