Add some unit tests for sled-agent Instance creation

[lifning]

Depends on #4325 for faking zone creation.

At time of writing, instance creation roughly looks like:

• nexus -> sled-agent: instance_put_state
  • sled-agent: InstanceManager::ensure_state
    • sled-agent: Instance::propolis_ensure
      • sled-agent -> nexus: cpapi_instances_put (if not migrating)
      • sled-agent: Instance::setup_propolis_locked (blocking!)
        • RunningZone::install and Zones::boot
        • illumos_utils::svc::wait_for_service
        • self::wait_for_http_server for propolis-server itself
      • sled-agent: Instance::ensure_propolis_and_tasks
        • sled-agent: spawn Instance::monitor_state_task
      • sled-agent -> nexus: cpapi_instances_put (if not migrating)
    • sled-agent: return ok result
• nexus: handle_instance_put_result

Or at least, it does in the happy path. #3927 saw propolis zone
creation take longer than the minute nexus's call to sled-agent's
instance_put_state. That might've looked something like:

• nexus -> sled-agent: instance_put_state
  • sled-agent: InstanceManager::ensure_state
    • sled-agent: Instance::propolis_ensure
      • sled-agent -> nexus: cpapi_instances_put (if not migrating)
      • sled-agent: Instance::setup_propolis_locked (blocking!)
        • RunningZone::install and Zones::boot
• nexus: i've been waiting a whole minute for this. connection timeout! handle_instance_put_result
  • [...]
  • sled-agent: return... oh, they hung up. :(

To avoid this timeout being implicit at the Dropshot configuration
layer (that is to say, we should still have some timeout),
we could consider a small refactor to make instance_put_state not a
blocking call -- especially since it's already sending nexus updates on
its progress via out-of-band cpapi_instances_put calls! That might look
something like:

• nexus -> sled-agent: instance_put_state
  • sled-agent: InstanceManager::ensure_state
    • sled-agent: spawn {
      • sled-agent: Instance::propolis_ensure
        • sled-agent -> nexus: cpapi_instances_put (if not migrating)
        • sled-agent: Instance::setup_propolis_locked (blocking!)
        • sled-agent: Instance::ensure_propolis_and_tasks
          • sled-agent: spawn Instance::monitor_state_task
        • sled-agent -> nexus: cpapi_instances_put (if not migrating)
        • sled-agent -> nexus: a cpapi call equivalent to the handle_instance_put_result nexus currently invokes after getting the response from the blocking call

(With a way for nexus to cancel an instance creation by ID, and a timeout
in sled-agent itself for terminating the attempt and reporting the failure
back to nexus, and a shorter threshold for logging the event of an instance
creation taking a long time.)

Before such a change, though, we should really have some more tests around
sled-agent's instance creation code at all! So here's a few.

[lifning]

closing this because it makes more sense to just pull it in as part of #4691