test: use NATS in coderdtest by default (#27343)

Closes GRU-70

Enables NATS as the pubsub for `coderdtest` unless specifically overwritten by the test case.

enterprise/coderd/coderd.go

@@ -721,6 +721,12 @@ func New(ctx context.Context, options *Options) (_ *API, err error) {
 		return nil, xerrors.Errorf("mount scim routes: %w", mountScimError)
 	}
 
+	// The NATS pubsub, if enabled, used the Replica Manager for clustering. It's a layering violation if the pubsub
+	// for the Replica Manager *is* the NATS pubsub, because then we have a dependency loop.
+	if _, isNats := options.ReplicaSyncPubsub.(*nats.Pubsub); isNats {
+		return nil, xerrors.Errorf("replica sync pubsub cannot be the NATS pubsub")
+	}
+
 	// We always want to run the replica manager even if we don't have DERP
 	// enabled, since it's used to detect other coder servers for licensing,
 	// and NATS clustering for HA pubsub.