fix race condition between service class and manually mocked store values

spec/javascripts/issuable/related_issues/components/related_issues_root_spec.js

 import Vue from 'vue';
 import relatedIssuesRoot from '~/issuable/related_issues/components/related_issues_root.vue';
+import relatedIssuesService from '~/issuable/related_issues/services/related_issues_service.js';
 
 const defaultProps = {
   endpoint: '/foo/bar/issues/1/related_issues',
@@ -41,11 +42,17 @@ describe('RelatedIssuesRoot', () => {
 
   describe('methods', () => {
     describe('onRelatedIssueRemoveRequest', () => {
-      beforeEach(() => {
+      beforeEach((done) => {
+        spyOn(relatedIssuesService.prototype, 'fetchRelatedIssues').and.returnValue(Promise.reject());
+
         vm = new RelatedIssuesRoot({
           propsData: defaultProps,
         }).$mount();
-        vm.store.setRelatedIssues([issuable1]);
+
+        setTimeout(() => {
+          vm.store.setRelatedIssues([issuable1]);
+          done();
+        });
       });
 
       it('remove related issue and succeeds', (done) => {