[ci skip] Adds unit tests for issue_system_note component

spec/javascripts/notes/components/issue_system_note_spec.js

+import Vue from 'vue';
+import issueSystemNote from '~/notes/components/issue_system_note.vue';
+import store from '~/notes/stores';
+
 describe('issue system note', () => {
-  it('should render a list item with correct id', () => {
+  let vm;
+  let props;
+
+  beforeEach(() => {
+    props = {
+      note: {
+        id: 1424,
+        author: {
+          id: 1,
+          name: 'Root',
+          username: 'root',
+          state: 'active',
+          avatar_url: 'path',
+          path: '/root',
+        },
+        note_html: '<p dir="auto">closed</p>',
+        system_note_icon_name: 'icon_status_closed',
+        created_at: '2017-08-02T10:51:58.559Z',
+      },
+    };
 
+    store.dispatch('setTargetNoteHash', `note_${props.note.id}`);
+
+    const Component = Vue.extend(issueSystemNote);
+    vm = new Component({
+      store,
+      propsData: props,
+    }).$mount();
   });
 
-  it('should render target class is note is target note', () => {
+  it('should render a list item with correct id', () => {
+    expect(vm.$el.getAttribute('id')).toEqual(`note_${props.note.id}`);
+  });
 
+  it('should render target class is note is target note', () => {
+    expect(vm.$el.classList).toContain('target');
   });
 
   it('should render svg icon', () => {
-
+    expect(vm.$el.querySelector('.timeline-icon svg')).toBeDefined();
   });
 
   it('should render note header component', () => {
-
+    expect(
+      vm.$el.querySelector('.system-note-message').innerHTML,
+    ).toEqual(props.note.note_html);
   });
 });