Step 8.7: Introduce focus-visible import where needed to repair tests

There is an import in `MatrixChat` which no longer gets executed when we don't skin the entire app, so bring it in manually.

test/components/structures/ThreadPanel-test.tsx

@@ -16,6 +16,7 @@ limitations under the License.
 
 import React from 'react';
 import { shallow, mount } from "enzyme";
+import 'focus-visible'; // to fix context menus
 
 import {
     ThreadFilterType,