jcstress: All State objects should be processed

There is a reasonable expectation that once `@State` object is created, it would eventually be acted upon. In fact, the Javadoc for `@Actor`/`@Arbiter` mentions that actor methods would be called on each state object exactly once. There are cases where users rely on this implicit lifecycle assumption to perform cleanups at `@Arbiter` methods.

Unfortunately, once jcstress leaves the test, it keeps last created state object slice unprocessed, which breaks this assumption.