add extension to assert TaskCompletionSource - #1267

This is a new extension to create assertions on TaskCompletionSource<T> (TCS).

This PR creates a breaking change, because for instances of TCS the assertions of the currently catching ObjectAssertions extension will not be available anymore.
But FA is going to version 6.0, so it should be acceptable.

Further, I tried to analyze the assertions which may get lost. Here is my answer to the question "Do we need such assertion?":

• ObjectAssertions
  • Be/NotBe: No, why should we check equivalency?
  • BeEquivalentTo/NotBeEquivalentTo: No, why should we check equivalency?
• ReferenceTypeAssertions
  • BeNull/NotBeNull: Unsure, I do not see any relevance.
  • BeSameAs/NotBeSameAs: Unsure, could be relevant if TCS is part of an API.
  • BeOfType/NotBeOfType: No, I do not see any relevance.
  • BeAssignableTo/NotBeAssignableTo: No, I do not see any relevance.
  • Match: No, I do not see any relevance.

Please help to answer these questions and check whether you agree.

While creating unit tests using the clock API I was a bit confused whether the methods CompletesBeforeTimeout and RunsIntoTimeout influences the behavior of DelayAsync. I found, it did not. So I introduced new methods and changed existing usages. Hope you agree.
If not, we should think, whether DelayAsync or even more the IClock API can be adopted to work similar to FakeClock.Wait. Maybe DelayAsync should be replaced by a WaitAsync returning boolean too.