audio: run the tests waiting on real time on the fake clock of synctest

TestGC, TestSameSourcePlayers, TestDeferredDeviceCreation,
TestPauseAndStopReading, TestPauseAndStopReadingWhilePaused,
TestPositionNotGrowingAfterFinished and
TestPlayerErrorDoesNotStopOtherPlayers now run in a synctest bubble, so
their sleeps and deadlines take no real time.

A bubble must not leave goroutines behind, so ResetContextForTesting
now stops the goroutine updating the players and the dummy players
reading their sources, and waits for them to exit. Before, every test
leaked them.

Updates #3716

audio/audio.go

@@ -73,9 +73,17 @@ type Context struct {
 	playingPlayers map[*playerImpl]struct{}
 
 	m sync.Mutex
-	// cond is signalled under m when a player is added to playingPlayers. The goroutine
-	// updating the players waits on it while playingPlayers is empty.
+	// cond is signalled under m when a player is added to playingPlayers or when the context is
+	// disposed. The goroutine updating the players waits on it while playingPlayers is empty.
 	cond *sync.Cond
+
+	// disposed reports that the goroutine updating the players must exit. A context lives until the
+	// process exits, so this is set only when a test discards the context.
+	disposed bool
+
+	// updater tracks the goroutine updating the players, so that a test discarding the context can wait
+	// for the goroutine to exit.
+	updater sync.WaitGroup
 }
 
 var (
@@ -162,12 +170,14 @@ func NewContext(sampleRate int) *Context {
 	// In the current Ebitengine implementation, update might not be called when the window is in the background (#3154).
 	// In this case, an audio player's position is not updated correctly with AppendHookOnBeforeUpdateWithVMGuestInfo.
 	// Use a distinct goroutine to update the player states.
-	go func() {
+	c.updater.Go(func() {
 		for {
 			// Block while there is no player to update, so that an idle context does not wake
 			// up periodically. The sweep itself can remove the last player, so the check is
 			// repeated after every sweep.
-			c.waitForPlayingPlayers()
+			if !c.waitForPlayingPlayers() {
+				return
+			}
 			// A failure is local to the player which reported it. Record it and keep updating
 			// the other players (#3647).
 			if err := c.updatePlayers(); err != nil {
@@ -175,7 +185,7 @@ func NewContext(sampleRate int) *Context {
 			}
 			time.Sleep(time.Second / 100)
 		}
-	}()
+	})
 
 	return c
 }
@@ -212,13 +222,15 @@ func (c *Context) setReady() {
 	c.m.Unlock()
 }
 
-// waitForPlayingPlayers blocks until at least one player is registered as playing.
-func (c *Context) waitForPlayingPlayers() {
+// waitForPlayingPlayers blocks until at least one player is registered as playing, and reports
+// whether the players should be updated. It returns false once the context is disposed.
+func (c *Context) waitForPlayingPlayers() bool {
 	c.m.Lock()
 	defer c.m.Unlock()
-	for len(c.playingPlayers) == 0 {
+	for len(c.playingPlayers) == 0 && !c.disposed {
 		c.cond.Wait()
 	}
+	return !c.disposed
 }
 
 func (c *Context) addPlayingPlayer(p *playerImpl) {