Wait for the callbacks these tests assert on. (#4803)

TestUnsubscribe checked that the changed-notifier observer was gone as
soon as the unsubscribe had settled, but setDesired leaves the
RemoveObserver call to a goroutine of its own and nothing the test waits
on orders against it, so CI caught the assertion running first.
TestSubscribe has the same defect on the unsubscribed callback, which
unmarkSubscribedTo delivers with a bare go while the subscribed one is
called inline.

Wait for both, each with a message of its own, so that a leak that is
real still says which of them broke.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

pkg/rtc/subscriptionmanager_test.go

@@ -113,7 +113,10 @@ func TestSubscribe(t *testing.T) {
 		require.Eventually(t, func() bool {
 			return numParticipantSubscribed.Load() == 2
 		}, subSettleTimeout, subCheckInterval, "participant subscribe status was not updated twice")
-		require.Equal(t, int32(1), numParticipantUnsubscribed.Load())
+		// the unsubscribed callback is delivered on a goroutine of its own
+		require.Eventually(t, func() bool {
+			return numParticipantUnsubscribed.Load() == 1
+		}, subSettleTimeout, subCheckInterval, "participant unsubscribe status was not updated")
 	})
 
 	t.Run("no track permission", func(t *testing.T) {
@@ -248,7 +251,10 @@ func TestUnsubscribe(t *testing.T) {
 
 	// no traces should be left
 	require.Len(t, sm.GetSubscribedTracks(), 0)
-	require.False(t, res.TrackChangedNotifier.HasObservers())
+	// the observer is dropped on a goroutine of its own
+	require.Eventually(t, func() bool {
+		return !res.TrackChangedNotifier.HasObservers()
+	}, subSettleTimeout, subCheckInterval, "observer was not removed")
 
 	tl := sm.params.Participant.GetTelemetryListener().(*typesfakes.FakeParticipantTelemetryListener)
 	require.Equal(t, 1, tl.OnTrackUnsubscribedCallCount())