add waiting count

lib.go

@@ -58,3 +58,8 @@ func (c TestClock) Pass(d time.Duration) {
 func (c TestClock) Jump(t time.Time) {
 	c.clock.jump(t)
 }
+
+// Waiting tells how many channels are waiting set by After().
+func (c TestClock) Waiting() int {
+	return c.clock.waiting()
+}

test.go

@@ -100,3 +100,12 @@ func (t test) jump(to time.Time) {
 	s.current = to
 	s = triggerChans(s)
 }
+
+func (t test) waiting() int {
+	s := <-t.state
+	defer func() {
+		t.state <- s
+	}()
+
+	return len(s.chantlist)
+}