Add tstate check.

Author: ZeroIntensity

Modules/_testcapimodule.c

@@ -2617,9 +2617,11 @@ test_interp_ensure(PyObject *self, PyObject *unused)
     PyInterpreterRef sub_ref = get_strong_ref();
     PyInterpreterState *subinterp = PyThreadState_GetInterpreter(tstate);
 
+    assert(PyThreadState_GetUnchecked() == NULL);
     for (int i = 0; i < 10; ++i) {
         int res = PyThreadState_Ensure(ref);
         assert(res == 0);
+        assert(PyThreadState_GetUnchecked() != NULL);
         assert(PyInterpreterState_Get() == interp);
     }