allocrunner: handle alloc updates from job stop.

[jrasell]

This change adds minor functionality to handle job stops which are handled within the alloc-runner by the update function. When an allocation update is pulled from the servers, the simulated runner can now handle transitions to stop by correctly modifying alloc state parameters.

[lgfa29]

Ah funny, didn't realize that the simulated nodes don't actually run anything. This explains why none of the mock driver configs I tried actually worked 😅

I wonder if we'll need to make it work for some tests, but we can do that when the need arises.