Fix intermittent notifier stress test

Here, fix an intermittent stress test in the notifier package. This is
one that I added last minute in #258 in response to code review, so I
missed the intermittency because it didn't fail on any of the last
pushes that I made, but I noticed it failing today [1].

Luckily, it's quite a simple where we just have to make sure that the
test waits for one of the test goroutines to return before quitting. Ran
at `-race -count 1000` locally and no longer seeing any intermittency.

[1] https://github.com/riverqueue/river/actions/runs/8223455278/job/22486174354

internal/notifier/notifier_test.go

@@ -410,9 +410,14 @@ func TestNotifier(t *testing.T) {
 			notifyChans[i] = make(chan TopicAndPayload, 1000)
 		}
 
+		var wg sync.WaitGroup
+
 		// Start a goroutine to send messages constantly.
 		shutdown := make(chan struct{})
+		wg.Add(1)
 		go func() {
+			defer wg.Done()
+
 			ticker := time.NewTicker(10 * time.Millisecond)
 			defer ticker.Stop()
 
@@ -430,7 +435,6 @@ func TestNotifier(t *testing.T) {
 			}
 		}()
 
-		var wg sync.WaitGroup
 		wg.Add(len(notifyChans))
 		for i := range notifyChans {
 			notifyChan := notifyChans[i]
@@ -443,15 +447,15 @@ func TestNotifier(t *testing.T) {
 					require.NoError(t, err)
 
 					// Pause a random brief amount of time.
-					notifier.BaseService.CancellableSleepRandomBetween(ctx, 5*time.Millisecond, 50*time.Millisecond)
+					notifier.BaseService.CancellableSleepRandomBetween(ctx, 15*time.Millisecond, 50*time.Millisecond)
 
 					sub.Unlisten(ctx)
 				}
 			}()
 		}
 
-		wg.Wait()
 		close(shutdown)
+		wg.Wait()
 		notifier.Stop()
 
 		for i := range notifyChans {