Fix test_capi

Author: vstinner

Modules/_testinternalcapi.c

@@ -2437,7 +2437,7 @@ test_threadstate_set_stack(PyObject *self, PyObject *Py_UNUSED(args))
     PyThreadState *tstate = PyThreadState_GET();
 
     // Test a size smaller than _PyOS_STACK_MARGIN_BYTES
-    size_t size = 123;
+    size_t size = 4096;
     assert(size < _PyOS_STACK_MARGIN_BYTES);
     void *start = (void*)(_Py_get_machine_stack_pointer() - size);
     check_threadstate_set_stack(tstate, start, size);