Add unit test to ensure a connection has been properly closed

[scamille]

If canceled during data transmission, #349 adds code to ensure that the connection automatically gets canceled.

Add a test to ensure this works.
Also, add an inverted version to ensure that cancellation before sending data does not result in cancellation.

[scamille]

Works on my machine :-/

[mycroes]

I get the same error locally, pay attention that it only happens on net452, not the other targets.

[mycroes]

Seems to me the issue is with cancellationSource.CancelAfter(TimeSpan.FromMilliseconds(5)). Perhaps sometimes that triggers before the actual method on net472. Not sure why that's included in both tests (I think it shouldn't be in either...).

[mycroes]

The fact that the tests now fail on a single platform is really good though. I still only have AppVeyor as required check, but I'm considering to make them all required.

[scamille]

Right I thought I removed that line, that of course explains it, thanks.

It might sense to wait a bit more to see how reliable the new CI actions are. On the other I do know from the simulation craft/simc project that they are pretty reliable, since we have a lot more commits and longer tests over there, but not any .NET.
Macos jobs can sometimes fail but it is still rare.

[mycroes]

Thanks a lot @scamille!