Add TestSubscriber

[zsxwing]

This PR adds TestSubscriber to RxScala.

There are two differences between RxJava TestSubscriber and RxScala TestSubscriber.

• RxScala uses a singleton Notification.OnCompleted, so instead of adding getOnCompletedEvents to RxScala, I add getOnCompletedEventsCount that just return the number of Notification.OnCompleteds.
• RxJava has two similar methods: assertReceivedOnNext(List<T> items) and assertValues(T... values). Since they are almost same exception the parameter type, I only add assertValues(T... values) to RxScala.

Question:

• Do we need to add TestObserver? Is there a case that TestObserver is more useful than TestSubscriber?
• Better name for getOnCompletedEventsCount?

[zsxwing]

cc @samuelgruetter

[samuelgruetter]

• Do we need to add TestObserver? Is there a case that TestObserver is more useful than TestSubscriber?

I can't think of any such case. Let's not add it until some users request it.

• Better name for getOnCompletedEventsCount?

I would not add this method, nor getOnCompletedEvents. Instead, one should use assertCompleted and assertNotCompleted.

[zsxwing]

I would not add this method, nor getOnCompletedEvents. Instead, one should use assertCompleted and assertNotCompleted.

Agree. Removed it.

[samuelgruetter]

LGTM