use old sync.WaitGroup API in tests

Author: marten-seemann

v2_encode_test.go

@@ -1415,14 +1415,16 @@ func TestIssue63379(t *testing.T) {
 func TestSynctestMarshal(t *testing.T) {
 	var wg sync.WaitGroup
 	for range 5 {
-		wg.Go(func() {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
 			synctest.Test(t, func(t *testing.T) {
 				_, err := Marshal([]string{})
 				if err != nil {
 					t.Errorf("Marshal: %v", err)
 				}
 			})
-		})
+		}()
 	}
 	wg.Wait()
 }