Fix so that test.test_distutils can be executed by unittest and not just regrtest (GH-13480)

Lib/test/test_distutils.py

@@ -10,9 +10,15 @@ import test.support
 
 
 def test_main():
+    # used by regrtest
     test.support.run_unittest(distutils.tests.test_suite())
     test.support.reap_children()
 
 
+def load_tests(*_):
+    # used by unittest
+    return distutils.tests.test_suite()
+
+
 if __name__ == "__main__":
     test_main()