pythongh-122957: Fix test flakiness in asyncio test in free-thread bu…

…ild (python#124039)

Lib/test/test_asyncio/test_threads.py

@@ -30,15 +30,17 @@ async def test_to_thread_once(self):
         func.assert_called_once()
 
     async def test_to_thread_concurrent(self):
-        func = mock.Mock()
+        calls = []
+        def func():
+            calls.append(1)
 
         futs = []
         for _ in range(10):
             fut = asyncio.to_thread(func)
             futs.append(fut)
         await asyncio.gather(*futs)
 
-        self.assertEqual(func.call_count, 10)
+        self.assertEqual(sum(calls), 10)
 
     async def test_to_thread_args_kwargs(self):
         # Unlike run_in_executor(), to_thread() should directly accept kwargs.