test(filter): enable type inference in marble diagram

- relates to ReactiveX#1633

Author: kwonoj

spec/operators/filter-spec.ts

@@ -1,6 +1,12 @@
 import {expect} from 'chai';
 import * as Rx from '../../dist/cjs/Rx';
-declare const {hot, cold, asDiagram, expectObservable, expectSubscriptions};
+import marbleTestingSignature = require('../helpers/marble-testing'); // tslint:disable-line:no-require-imports
+
+declare const { asDiagram };
+declare const hot: typeof marbleTestingSignature.hot;
+declare const cold: typeof marbleTestingSignature.cold;
+declare const expectObservable: typeof marbleTestingSignature.expectObservable;
+declare const expectSubscriptions: typeof marbleTestingSignature.expectSubscriptions;
 
 const Observable = Rx.Observable;