Fix flaky spec caused by missing stub return value

vm.loadList() is called in the mounted method of
board_list_selector, so when the component is mounted in
beforeEach, then loadList gets called and assignees list
_is_ empty. There is probably something worse going on
underneath here but this seems like an easy safe workaround

ee/spec/javascripts/boards/components/board_list_selector/board_list_selector_spec.js

@@ -58,7 +58,7 @@ describe('BoardListSelector', () => {
       });
 
       it('does not call axios.get when store.state.assignees is not empty', done => {
-        spyOn(axios, 'get');
+        spyOn(axios, 'get').and.returnValue(Promise.resolve());
         boardsStore.state.assignees = mockAssigneesList;
 
         vm.loadList()