[3.12] pythongh-76785: Make test.test_interpreters executable (pythonGH-112982)

This is so that we can run python -m test.test_interpreters. As such it
backports that aspect of commit 86a77f4,
where it is implemnented by a package __main__.py.

Author: jeff5

Lib/test/test_interpreters.py

@@ -1053,3 +1053,7 @@ def test_recv_nowait_default(self):
         self.assertEqual(obj4, b'spam')
         self.assertEqual(obj5, b'eggs')
         self.assertIs(obj6, default)
+
+
+if __name__ == "__main__":
+    unittest.main()