[3.11] gh-76785: Make test.test_interpreters executable (GH-112982) (GH-113470) (GH-113473)

This is so that we can run python -m test.test_interpreters. As such it
backports that aspect of commit 86a77f4e1a,
where it is implemented by a package __main__.py.
(cherry picked from commit bdad5c367f)

Co-authored-by: Jeff Allen <ja.py@farowl.co.uk>

Lib/test/test_interpreters.py

@@ -745,3 +745,7 @@ class TestSendRecv(TestBase):
         self.assertEqual(obj4, b'spam')
         self.assertEqual(obj5, b'eggs')
         self.assertIs(obj6, default)
+
+
+if __name__ == "__main__":
+    unittest.main()