@Test
    public void testConfigureOnlyOnceUponSubscribe() {
        final String configId = "raw:times 1\n";
        AppService service1 = new AppService(configId, sourceSet);
        AppService service2 = new AppService(configId, sourceSet);

        assertEquals(1, service1.timesConfigured());
        assertEquals(1, service2.timesConfigured());

        service1.cancelSubscription();
        service2.cancelSubscription();
    }
Verifies that the subscriber's configure() method is only
called once upon subscribe, even if there are more than one
subscribers to the same ConfigInstance. This has previously
been a problem, since ConfigInstance.subscribe() called
configureSubscriber(), which configures all subscribers to the
instance. Now, the new method configureSubscriber(Subscriber)
is called instead.
config/src/test/java/com/yahoo/config/subscription/ConfigInstanceTest.java