In polygerrit tests import sinon directly

Our current hack in common-test-setup.ts that adds the name to global
works, but leaves linter complaining about "symbol not found"

Release-Notes: skip
Change-Id: Ie13f1b336a8454f668893da81e6887647e0dd4b9
(cherry picked from commit ac4c1888e9cb5ad51dffc0e5cd58ca067f189b18)
diff --git a/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts b/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
index 4a78140..6a86ad1 100644
--- a/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
+++ b/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
@@ -3,6 +3,7 @@
  * Copyright 2017 Google LLC
  * SPDX-License-Identifier: Apache-2.0
  */
+import * as sinon from 'sinon';
 import '../../../test/common-test-setup';
 import './gr-router';
 import {Page, PageContext} from './gr-page';