async void methods should be considered as “Fire and Forget” – there is no way to wait for them to finish. If Visual Studio were to start one of these tests, it wouldn’t be able to wait for the test to complete (mark it as successful) or trap any exceptions raised.
With an async Task, the caller is able to wait for execution to complete, and to trap any exceptions raised while it runs.
See this answer for more discussion of async void vs async Task.