audio: run TestPlayOnIdleContext on the fake clock of synctest

The test was added after the survey in #3716, and waits on real time in
the same way.

Updates #3716

audio/audio_test.go

@@ -879,6 +879,7 @@ func TestPlayerErrorDoesNotStopOtherPlayers(t *testing.T) {
 }
 
 func TestPlayOnIdleContext(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
 		setup()
 		defer teardown()
 
@@ -888,7 +889,7 @@ func TestPlayOnIdleContext(t *testing.T) {
 		}
 
 		// waitUntil polls cond until it holds or the deadline passes, and reports whether it held.
-	// The deadline is generous because time.Sleep has a several-millisecond floor on browsers.
+		// The test runs on the fake clock of synctest, so the deadline does not take real time.
 		waitUntil := func(cond func() bool) bool {
 			deadline := time.Now().Add(10 * time.Second)
 			for time.Now().Before(deadline) {
@@ -925,6 +926,7 @@ func TestPlayOnIdleContext(t *testing.T) {
 				t.Fatalf("round %d: time out: the player did not finish", i)
 			}
 		}
+	})
 }
 
 func callWithTimeout(t *testing.T, name string, f func() error) error {