test(combineLatest): add unsubscription-related tests
test(combineLatest): add unsubscription-related tests

Add tests for combineLatest, to verify it behaves correctly when the result is explicitly
unsubscribed.

staltz/RxJSNext,adrianomelo/RxJSNext,ReactiveX/rxjs,staltz/RxJSNext,ReactiveX/RxJS,SangKa/RxJS-Docs-CN,elliotschi/rxjs,martinsik/rxjs,Igorbek/rxjs5,RxJS-CN/RxJS-Docs-CN,adrianomelo/RxJS-1,staltz/RxJSNext,robwormald/RxJS,gluck/RxJS,martinsik/rxjs,martinsik/rxjs,jadbox/RxJS,adrianomelo/RxJSNext,justinwoo/RxJS,ReactiveX/RxJS,RxJS-CN/RxJS-Docs-CN,figueredo/RxJS,martinsik/rxjs,benlesh/RxJS,ReactiveX/RxJS,RxJS-CN/RxJS-Docs-CN,saneyuki/RxJS,robwormald/RxJS,adrianomelo/RxJSNext,figueredo/RxJS,figueredo/RxJS,figueredo/RxJS,trxcllnt/RxJS,benlesh/RxJS,adrianomelo/RxJS-1,elliotschi/rxjs,Brocco/RxJS,elliotschi/rxjs,Igorbek/rxjs5,jinroh/RxJS,saneyuki/RxJS,adrianomelo/RxJS-1,kwonoj/RxJS,jayphelps/rxjs,jayphelps/rxjs,trxcllnt/RxJS,staltz/RxJSNext,robwormald/RxJS,kwonoj/RxJS,Igorbek/rxjs5,IgorMinar/RxJS,justinwoo/RxJS,jayphelps/rxjs,SangKa/RxJS-Docs-CN,gluck/RxJS,gluck/RxJS,Igorbek/rxjs5,SangKa/RxJS-Docs-CN,ReactiveX/rxjs,ReactiveX/rxjs,elliotschi/rxjs,ReactiveX/rxjs,IgorMinar/RxJS,trxcllnt/RxJS,jadbox/RxJS,Brocco/RxJS,Brocco/RxJS,jayphelps/rxjs,kwonoj/RxJS,robwormald/RxJS,jayphelps/rxjs,SangKa/RxJS-Docs-CN,justinwoo/RxJS,RxJS-CN/RxJS-Docs-CN,benlesh/RxJS,jinroh/RxJS,trxcllnt/RxJS,jadbox/RxJS,justinwoo/RxJS,gluck/RxJS,jadbox/RxJS,jinroh/RxJS,benlesh/RxJS,adrianomelo/RxJSNext,kwonoj/RxJS,ReactiveX/RxJS,jinroh/RxJS,adrianomelo/RxJS-1,Brocco/RxJS