in order to simulate cases where propolis-server gets stuck or takes a long time between instance state transitions (e.g. #841), it would be nice if there was a way for tests to control the state transitions simulated by the mock server. this incldues including preventing them from occurring or delaying them until the test requests that they proceed, or injecting failures. this would be useful for testing sled-agent interactions with Propolis, such as oxidecomputer/omicron#7548