gh-149879: Fix test_capi on Cygwin

Fix Test_Pep523AllowSpecialization tests of test_capi.test_misc.

On Cygwin, _PyEval_EvalFrameDefault in _testinternalcapi is not the
same as _PyEval_EvalFrameDefault in python.exe. So pass NULL
explicitly to use the default function (_PyEval_EvalFrameDefault).

Author: vstinner

Modules/_testinternalcapi.c

@@ -1105,7 +1105,7 @@ static PyObject *
 set_eval_frame_default(PyObject *self, PyObject *Py_UNUSED(args))
 {
     module_state *state = get_module_state(self);
-    _PyInterpreterState_SetEvalFrameFunc(_PyInterpreterState_GET(), _PyEval_EvalFrameDefault);
+    _PyInterpreterState_SetEvalFrameFunc(_PyInterpreterState_GET(), NULL);
     Py_CLEAR(state->record_list);
     Py_RETURN_NONE;
 }