app/vmalert: use consistent randomizer in tests

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3683

app/vmalert/manager_test.go

@@ -64,17 +64,18 @@ func TestManagerUpdateConcurrent(t *testing.T) {
 	wg := sync.WaitGroup{}
 	wg.Add(workers)
 	for i := 0; i < workers; i++ {
-		go func() {
+		go func(n int) {
 			defer wg.Done()
+			r := rand.New(rand.NewSource(int64(n)))
 			for i := 0; i < iterations; i++ {
-				rnd := rand.Intn(len(paths))
+				rnd := r.Intn(len(paths))
 				cfg, err := config.Parse([]string{paths[rnd]}, notifier.ValidateTemplates, true)
 				if err != nil { // update can fail and this is expected
 					continue
 				}
 				_ = m.update(context.Background(), cfg, false)
 			}
-		}()
+		}(i)
 	}
 	wg.Wait()
 }

app/vmalert/notifier/config_watcher_test.go

@@ -162,14 +162,15 @@ consul_sd_configs:
 	wg := sync.WaitGroup{}
 	wg.Add(workers)
 	for i := 0; i < workers; i++ {
-		go func() {
+		go func(n int) {
 			defer wg.Done()
+			r := rand.New(rand.NewSource(int64(n)))
 			for i := 0; i < iterations; i++ {
-				rnd := rand.Intn(len(paths))
+				rnd := r.Intn(len(paths))
 				_ = cw.reload(paths[rnd]) // update can fail and this is expected
 				_ = cw.notifiers()
 			}
-		}()
+		}(i)
 	}
 	wg.Wait()
 }

app/vmalert/remotewrite/remotewrite_test.go

@@ -27,12 +27,13 @@ func TestClient_Push(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to create client: %s", err)
 	}
+	r := rand.New(rand.NewSource(1))
 	const rowsN = 1e4
 	var sent int
 	for i := 0; i < rowsN; i++ {
 		s := prompbmarshal.TimeSeries{
 			Samples: []prompbmarshal.Sample{{
-				Value:     rand.Float64(),
+				Value:     r.Float64(),
 				Timestamp: time.Now().Unix(),
 			}},
 		}