[pseudoIDs] Fixes for room alias tests (#3159)

Some (deceptively) simple fixes for some bugs that caused room alias
tests to fail (sytext `tests/30rooms/05aliases.pl`). Each commit has
details about what it fixes.

Sytest results:

- Sytest before (79d4a0e):
https://gist.github.com/swedgwood/972ac4ef93edd130d3db0930703d6c82
- Sytest after (4b09bed):
https://gist.github.com/swedgwood/504b00ac4ee892acb757b7fac55fa28a

Room aliases go from `8/15` to `15/15`, but looks like these fixes also
managed to fix about `4` other tests, which is a nice bonus :)

Signed-off-by: `Sam Wedgwood <sam@wedgwood.dev>`

roomserver/internal/perform/perform_create_room.go

@@ -433,23 +433,16 @@ func (c *Creator) PerformCreateRoom(ctx context.Context, userID spec.UserID, roo
 	// from creating the room but still failing due to the alias having already
 	// been taken.
 	if roomAlias != "" {
-		aliasReq := api.SetRoomAliasRequest{
-			Alias:  roomAlias,
-			RoomID: roomID.String(),
-			UserID: userID.String(),
-		}
-
-		var aliasResp api.SetRoomAliasResponse
-		err = c.RSAPI.SetRoomAlias(ctx, &aliasReq, &aliasResp)
-		if err != nil {
-			util.GetLogger(ctx).WithError(err).Error("aliasAPI.SetRoomAlias failed")
+		aliasAlreadyExists, aliasErr := c.RSAPI.SetRoomAlias(ctx, senderID, roomID, roomAlias)
+		if aliasErr != nil {
+			util.GetLogger(ctx).WithError(aliasErr).Error("aliasAPI.SetRoomAlias failed")
 			return "", &util.JSONResponse{
 				Code: http.StatusInternalServerError,
 				JSON: spec.InternalServerError{},
 			}
 		}
 
-		if aliasResp.AliasExists {
+		if aliasAlreadyExists {
 			return "", &util.JSONResponse{
 				Code: http.StatusBadRequest,
 				JSON: spec.RoomInUse("Room alias already exists."),