public void onlyOneTimerIsStarted() {
Monitor m1 = mock(Monitor.class);
when(m1.start("foo")).thenReturn(new Monitor.Timer.NULL());
Monitor m2 = mock(Monitor.class);
Monitor.Timer t2 = mock(Monitor.Timer.class);
when(m2.start("foo")).thenReturn(t2);
Monitor m3 = mock(Monitor.class);
when(m3.start("foo")).thenReturn(new Monitor.Timer.NULL());
CompositeMonitor monitor = new CompositeMonitor(m1, m2, m3);
Monitor.Timer chosenTimer = monitor.start("foo");