Provide a way for tests to inject 'exited' events into libprocess Processes.

This has come up a few times when trying to write tests; we often need to inject 'exited' events to simulate certain scenarios for testing. We don't currently have the means to do this, outside of terminating the linked libprocess Process.

There have been situations that we've observed in practice that we'd like to test in which we need to inject an 'exited' event.