Add simulate-until-command to support simultaneity tests

[karalekas]

For example, it would be great to test fully-rewinding multireweighting, but there's no way that case will be reached just by setting up a simple supervisor test. This is because a zero-weight edge will be seen earlier in the multireweighting process, causing it to abort. We would need to simulate until reaching MULTIREWEIGHT-BROADCAST-REWEIGHT, then reach in and modify the weight of a nearby node so that there is then a weightless edge between it and the HOLD cluster, and then simulate until dead. I think this feature will also allow us to write tests for SCANing -- we could inject the START-SCAN command and simulate until IDLE.

(defun simulate-until-command (simulation process command &key (start-time 0) timeout)
  "Runs `SIMULATION' until `PROCESS' reaches `COMMAND'."
  (loop :with stopping-time := start-time
        :with delta := (/ (anatevka::process-clock-rate process))
        :while (and (process-command-stack process)
                    (not (= command (anatevka::peek (process-command-stack process)))))
        :when
          (not (process-command-stack process))
          :do (error "SIMULATE-UNTIL-COMMAND supervisor died.")
        :when (and timeout (< timeout (- stopping-time start-time)))
          :do (error "SIMULATE-UNTIL-COMMAND hit a timeout.")
        :do (incf stopping-time delta)
            (simulation-run simulation :until stopping-time)
        :finally (return (values simulation stopping-time))))

[ecpeterson]

Canaries now make this pretty easy. (What isn't trivial is writing the test!)