Add some unit tests for sled-agent Instance 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. omicron#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!
- nexus: `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.

sled-agent/src/fakes/nexus.rs

@@ -8,13 +8,18 @@
 //! to operate correctly.
 
 use dropshot::{
-    endpoint, ApiDescription, FreeformBody, HttpError, HttpResponseOk, Path,
-    RequestContext,
+    endpoint, ApiDescription, FreeformBody, HttpError, HttpResponseOk,
+    HttpResponseUpdatedNoContent, Path, RequestContext, TypedBody,
 };
 use hyper::Body;
 use internal_dns::ServiceName;
 use omicron_common::api::external::Error;
-use omicron_common::api::internal::nexus::UpdateArtifactId;
+use omicron_common::api::internal::nexus::{
+    SledInstanceState, UpdateArtifactId,
+};
+use schemars::JsonSchema;
+use serde::Deserialize;
+use uuid::Uuid;
 
 /// Implements a fake Nexus.
 ///
@@ -28,6 +33,13 @@ pub trait FakeNexusServer: Send + Sync {
     ) -> Result<Vec<u8>, Error> {
         Err(Error::internal_error("Not implemented"))
     }
+    fn cpapi_instances_put(
+        &self,
+        _instance_id: Uuid,
+        _new_runtime_state: SledInstanceState,
+    ) -> Result<(), Error> {
+        Err(Error::internal_error("Not implemented"))
+    }
 }
 
 /// Describes the server context type.
@@ -52,9 +64,32 @@ async fn cpapi_artifact_download(
     ))
 }
 
+#[derive(Deserialize, JsonSchema)]
+struct InstancePathParam {
+    instance_id: Uuid,
+}
+
+#[endpoint {
+    method = PUT,
+    path = "/instances/{instance_id}",
+}]
+async fn cpapi_instances_put(
+    request_context: RequestContext<ServerContext>,
+    path_params: Path<InstancePathParam>,
+    new_runtime_state: TypedBody<SledInstanceState>,
+) -> Result<HttpResponseUpdatedNoContent, HttpError> {
+    let context = request_context.context();
+    context.cpapi_instances_put(
+        path_params.into_inner().instance_id,
+        new_runtime_state.into_inner(),
+    )?;
+    Ok(HttpResponseUpdatedNoContent())
+}
+
 fn api() -> ApiDescription<ServerContext> {
     let mut api = ApiDescription::new();
     api.register(cpapi_artifact_download).unwrap();
+    api.register(cpapi_instances_put).unwrap();
     api
 }