Implement analyzer to warn for use of Assert methods in async void

[Youssef1313]

Fixes #4613

[UliKu-philips]

ReturnsVoid seems to be true also for async Task methods...
(getting the new warning now in .net framework 4.8 project for async Task TestMethod())

[Youssef1313]

@UliKu-philips Could you please open an issue for that, with a repro? I'm not able to reproduce the behavior you describe.

[UliKu-philips]

actually I think I am wrong... in the example here, triggering MSTEST0040 is caused by the async lambda. The Subscribe method takes an Action<T> which is void:

        [TestMethod]
        public async Task TestMethodAsync()
        {
            using (var waitEvent = new ManualResetEventSlim())
            {
                using (Observable.Timer(dueTime: TimeSpan.FromSeconds(0), period: TimeSpan.FromSeconds(1)).Subscribe(
                    async t =>
                    {
                        await Task.Delay(1);
                        Assert.IsTrue(t > 0);
                        waitEvent.Set();
                    }))
                {
                    waitEvent.Wait(TimeSpan.FromSeconds(2));
                    await Task.Delay(1);
                }
            }
        }

https://github.com/UliKu-philips/UnitTestAsyncTaskWarning/blob/main/UnitTestProject1/UnitTest1.cs

Thanks for the great work!