More explicit setting of global DOM function

Our component fails due to this method being undefined, so we mock it
out for these tests. We do not need to reset the mock as it is assigned
fresh on every test.

Author: rylnd

x-pack/plugins/security_solution/public/detections/components/value_lists_management_modal/auto_download.test.tsx

@@ -11,13 +11,10 @@ import { AutoDownload } from './auto_download';
 
 describe('AutoDownload', () => {
   beforeEach(() => {
+    // our DOM environment lacks this function that our component needs
     window.URL.revokeObjectURL = jest.fn();
   });
 
-  afterEach(() => {
-    (window.URL.revokeObjectURL as jest.Mock).mockReset();
-  });
-
   it('calls onDownload once if a blob is provided', () => {
     const onDownload = jest.fn();
     mount(<AutoDownload blob={new Blob([''])} onDownload={onDownload} />);