verify OnRunCompleted waits for pipeline to complete

[drewburlingame]

an attempt to verify #488.

@Sibusten can you post a failing test or point to a repo where this is failing? I updated a test that seems to verify OnRunCompleted is called only after the pipeline is executed. I might be missing something.

[Sibusten]

Here's a reproduction of the issue which shows disposal before the command finishes https://github.com/Sibusten/commanddotnet-488-repro

If you change the command in the test to async Task Do and await the Task.Delay you should see the issue. Right now the test is not waiting for the delay to finish because that task is not being awaited.