message: fix(TestScheduler): helper methods return proper types, `HotObservable` and `ColdObservable` instead of Observable
  Return `HotObservable` and `ColdObservable` instead of simply `Observable`, in order to give users access to subscriptions parameter
diff: import { Observable } from '../Observable';
  import { Notification } from '../Notification';
  -import { Subject } from '../Subject';
  import { ColdObservable } from './ColdObservable';
  import { HotObservable } from './HotObservable';
  import { TestMessage } from './TestMessage';
  @@ -36,7 +35,7 @@ export class TestScheduler extends VirtualTimeScheduler {
  return indexOf * TestScheduler.frameTimeFactor;
  }
  - createColdObservable<T>(marbles: string, values?: any, error?: any): Observable<T> {
  + createColdObservable<T>(marbles: string, values?: any, error?: any): ColdObservable<T> {
  if (marbles.indexOf('^') !== -1) {
  throw new Error('cold observable cannot have subscription offset "^"');
  }
  @@ -49,7 +48,7 @@ export class TestScheduler extends VirtualTimeScheduler {
  return cold;
  }
  - createHotObservable<T>(marbles: string, values?: any, error?: any): Subject<T> {
  + createHotObservable<T>(marbles: string, values?: any, error?: any): HotObservable<T> {
  if (marbles.indexOf('!') !== -1) {
  throw new Error('hot observable cannot have unsubscription marker "!"');
  }
  