all: use sync.WaitGroup.Go

Drops the Add/Done bookkeeping, which in batcher.go was spread across
three functions.

hscontrol/servertest/connect_race_test.go

@@ -96,22 +96,16 @@ func TestConnectDisconnectRace(t *testing.T) {
 
 		start := make(chan struct{})
 
-		wg.Add(2)
-
-		go func() {
-			defer wg.Done()
-
+		wg.Go(func() {
 			<-start
 
 			_, _ = srv.State().Disconnect(r2ID, gen)
-		}()
-		go func() {
-			defer wg.Done()
-
+		})
+		wg.Go(func() {
 			<-start
 
 			_, _ = srv.State().Connect(r2ID)
-		}()
+		})
 
 		close(start)
 		wg.Wait()

hscontrol/servertest/race_test.go

@@ -494,16 +494,12 @@ func TestRaceBatcherContention(t *testing.T) {
 			srv.App.Change(c)
 		})
 
-		wg.Add(1)
-
 		var c2 *servertest.TestClient
 
-		go func() {
-			defer wg.Done()
-
+		wg.Go(func() {
 			c2 = servertest.NewClient(t, srv, "rtcon-node2",
 				servertest.WithUser(user))
-		}()
+		})
 
 		wg.Wait()