Skip subinterpreter init test unless GIL disabled

Use an exec wrapper for subinterpreter execution and assert None results
instead of calling support.run_in_subinterp

Author: ashm-dev

Lib/test/test_struct.py

@@ -800,16 +800,19 @@ def test_c_complex_round_trip(self):
                     round_trip = struct.unpack(f, struct.pack(f, z))[0]
                     self.assertComplexesAreIdentical(z, round_trip)
 
+    @unittest.skipUnless(support.Py_GIL_DISABLED, 'this test can only possibly fail with GIL disabled')
     def test_endian_table_init_subinterpreters(self):
         # Verify that the _struct extension module can be initialized
         # concurrently in subinterpreters (gh-140260).
         from concurrent.futures import InterpreterPoolExecutor
 
+        def _exec_code(code_str):
+            exec(code_str)
+
         code = "import struct"
         with InterpreterPoolExecutor(max_workers=5) as executor:
-            results = executor.map(support.run_in_subinterp, [code] * 5)
-            for ret in results:
-                self.assertEqual(ret, 0)
+            results = executor.map(_exec_code, [code] * 5)
+            self.assertListEqual(list(results), [None] * 5)
 
 
 class UnpackIteratorTest(unittest.TestCase):