The class TaskCompletionSource<T> is quite useful in unit tests on asynchronous calls. The following assertions helps to check that the result is available within specific time.

var tcs = new TaskCompletionSource<bool>();
await tcs.Should().CompleteWithinAsync(1.Seconds());

The assertion returns the result for subsequent value assertions.

var tcs = new TaskCompletionSource<bool>();
await tcs.Should().CompleteWithinAsync(1.Seconds()).WithResult(true);

Additionally it is possible to assert that the task will not complete within specific time.

var tcs = new TaskCompletionSource<bool>();
await tcs.Should().NotCompleteWithinAsync(1.Seconds());