Add specs for Vue code

spec/frontend/jira_connect/components/app_spec.js

+import { shallowMount } from '@vue/test-utils';
+
+import JiraConnectApp from '~/jira_connect/components/app.vue';
+
+describe('JiraConnectApp', () => {
+  let wrapper;
+
+  const createComponent = ({ ...options }) => {
+    wrapper = shallowMount(JiraConnectApp, {
+      ...options,
+    });
+  };
+
+  afterEach(() => {
+    if (wrapper) {
+      wrapper.destroy();
+      wrapper = null;
+    }
+  });
+
+  const findHeader = () => wrapper.find('h3');
+  const findHeaderText = () => wrapper.find('h3').text();
+
+  describe('template', () => {
+    describe('newJiraConnectUi is false', () => {
+      it('does not render new UI', () => {
+        createComponent({
+          provide: {
+            glFeatures: { newJiraConnectUi: false },
+          },
+        });
+
+        expect(findHeader().exists()).toBe(false);
+      });
+    });
+
+    describe('newJiraConnectUi is true', () => {
+      it('renders new UI', () => {
+        createComponent({
+          provide: {
+            glFeatures: { newJiraConnectUi: true },
+          },
+        });
+
+        expect(findHeader().exists()).toBe(true);
+        expect(findHeaderText()).toBe('Linked namespaces');
+      });
+    });
+  });
+});