message: test(filter): enable type inference in marble diagram
  relates to
diff: 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;
  