Add repo_sidebar_spec

spec/javascripts/repo/repo_sidebar_spec.js

+import Vue from 'vue';
+import RepoStore from '~/repo/repo_store';
+import repoSidebar from '~/repo/repo_sidebar.vue';
+
+describe('RepoSidebar', () => {
+  function createComponent() {
+    const RepoSidebar = Vue.extend(repoSidebar);
+
+    return new RepoSidebar().$mount();
+  }
+
+  it('renders a list of tabs', () => {
+    const vm = createComponent();
+  });
+});