[Recipe] Process manager testing

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How to test a process manager produces the correct commands and state?

Use the process manager's handle/2 and apply/2 functions directly by reducing a list of one or more domain events. The resultant list of commands and process manager state can then be asserted on.

events = [%Event1{}, %Event2{}]
initial_state = %MyProcessManager{}

{commands, state} = 
  Enum.reduce(events, {[], initial_state}, fn event, {commands, state} ->
    new_commands = MyProcessManager.handle(state, event)
    new_state = MyProcessManager.apply(state, event)

    {commands ++ new_commands, new_state}
  end)

assert match?([%Command1{}, %Command2{}], commands)
assert match?(%MyProcessManager{key: value}, state)

This type of test can be run using async: true and will be very quick to execute as it does no IO.