Begin adding syncapi component tests (#2442)

* Add very basic syncapi tests

* Add a way to inject jetstream messages

* implement add_state_ids

* bugfixes

* Unbreak tests

* Remove now un-needed API call

* Linting

test/jetstream.go

@@ -0,0 +1,35 @@
+package test
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/setup/base"
+	"github.com/matrix-org/dendrite/setup/jetstream"
+	"github.com/nats-io/nats.go"
+)
+
+func MustPublishMsgs(t *testing.T, jsctx nats.JetStreamContext, msgs ...*nats.Msg) {
+	t.Helper()
+	for _, msg := range msgs {
+		if _, err := jsctx.PublishMsg(msg); err != nil {
+			t.Fatalf("MustPublishMsgs: failed to publish message: %s", err)
+		}
+	}
+}
+
+func NewOutputEventMsg(t *testing.T, base *base.BaseDendrite, roomID string, update api.OutputEvent) *nats.Msg {
+	t.Helper()
+	msg := &nats.Msg{
+		Subject: base.Cfg.Global.JetStream.Prefixed(jetstream.OutputRoomEvent),
+		Header:  nats.Header{},
+	}
+	msg.Header.Set(jetstream.RoomID, roomID)
+	var err error
+	msg.Data, err = json.Marshal(update)
+	if err != nil {
+		t.Fatalf("failed to marshal update: %s", err)
+	}
+	return msg
+}