fix: Wait for the executor.dispose done before exit the isolate

[littleGnAl]

We send a null to the sub isolate the clean up the native resources(destroyNativeApiEngine...), but we do not wait for the clean-up logic to be finished, which will cause an error if we want to clean up something relative to the native resources right after the dispose function in sychronize way. For example the test cases from the agora_rtc_engine https://github.com/AgoraIO-Extensions/Agora-Flutter-SDK/blob/main/test_shard/fake_test_app/integration_test/apis_call_fake_test.dart

  testWidgets(
    'AudioDeviceManager.enumeratePlaybackDevices',
    (WidgetTester tester) async {
      ...
      await rtcEngine.release();
    },
  );

  tearDown(() {
    setMockRtcEngineProvider(null);
    // It's not safe to dispose here, since the `executor.dispose` inside the isolate may run after this dispose function.
    irisTester!.dispose(); 
    irisTester = null;
  });