Add asyncio fixture to test_instance_async_method_spy (#516)

This ensures that this test executes and passes with pytest-8.4+.

pytest now throws errors [for such functions](pytest-dev/pytest#11372) which were skipped in older versions.

Author: kraj

tests/test_pytest_mock.py

@@ -524,6 +524,7 @@ def __call__(self, x):
     assert spy.spy_return_list == [20]
 
 
+@pytest.mark.asyncio
 async def test_instance_async_method_spy(mocker: MockerFixture) -> None:
     class Foo:
         async def bar(self, arg):