use import mocha instead of <reference> for some tests

extensions/emmet/src/test/updateImageSize.test.ts

@@ -3,6 +3,7 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 
+import 'mocha';
 import * as assert from 'assert';
 import { Selection } from 'vscode';
 import { withRandomFileEditor, closeAllEditors } from './testUtils';