Fix an incorrect test

The LMS app sends username not userid:

https://github.com/hypothesis/lms/blob/d6b88fd7e375a4b23899117556b3e39cfe18986b/lms/static/scripts/frontend_apps/components/LMSGrader.js#L36

But the test for ths client code that receives this was sending
userid.

Fix the test to send username, the same as the LMS app does.

src/sidebar/store/modules/test/selection-test.js

@@ -205,7 +205,7 @@ describe('sidebar/store/modules/selection', () => {
     it('sets the focused user and enables focus mode', function() {
       store.setFocusModeFocused(false);
       store.changeFocusModeUser({
-        userid: 'testuser',
+        username: 'testuser',
         displayName: 'Test User',
       });
       assert.equal(store.focusModeUserId(), 'testuser');