[ci skip] Adds tests for placeholder system note

app/assets/javascripts/notes/components/issue_placeholder_note.vue

 <script>
+  import userAvatarLink from '../../vue_shared/components/user_avatar/user_avatar_link.vue';
+
   export default {
+    name: 'issuePlaceholderNote',
     props: {
       note: {
         type: Object,
         required: true,
       },
     },
+    components: {
+      userAvatarLink,
+    },
     data() {
       return {
-        currentUser: window.gl.currentUserData,
+        currentUser: this.$store.getters.getUserData,
       };
     },
   };
@@ -18,12 +24,11 @@
   <li class="note being-posted fade-in-half timeline-entry">
     <div class="timeline-entry-inner">
       <div class="timeline-icon">
-        <a :href="currentUser.path">
-          <img
-            :src="currentUser.avatar_url"
-            class="avatar s40"
+        <user-avatar-link
+          :link-href="currentUser.path"
+          :img-src="currentUser.avatar_url"
+          :size="40"
           />
-        </a>
       </div>
       <div
         :class="{ discussion: !note.individual_note }"

app/assets/javascripts/notes/components/issue_placeholder_system_note.vue

 <script>
   export default {
+    name: 'placeholderSystemNote',
     props: {
       note: {
         type: Object,

spec/javascripts/notes/components/issue_placeholder_note_spec.js

+import Vue from 'vue';
+import placeholderNote from '~/notes/components/issue_placeholder_note.vue';
+
+describe('issue placeholder system note component', () => {
+  let mountComponent;
+  beforeEach(() => {
+    const PlaceholderNote = Vue.extend(placeholderNote);
+
+    mountComponent = props => new PlaceholderNote({
+      propsData: {
+        note: props,
+      },
+    }).$mount();
+  });
+
+  describe('user information', () => {
+    it('should render user avatar with link', () => {
+
+    });
+  });
+
+  describe('note content', () => {
+    it('should render note header information', () => {
+
+    });
+
+    it('should render note body', () => {
+
+    });
+
+    it('should render system note placeholder with markdown', () => {
+
+    });
+
+    it('should render emojis', () => {
+
+    });
+
+    it('should render slash commands', () => {
+
+    });
+  });
+});

spec/javascripts/notes/components/issue_placeholder_system_note_spec.js

+import Vue from 'vue';
+import placeholderSystemNote from '~/notes/components/issue_placeholder_system_note.vue';
+
+describe('issue placeholder system note component', () => {
+  let mountComponent;
+  beforeEach(() => {
+    const PlaceholderSystemNote = Vue.extend(placeholderSystemNote);
+
+    mountComponent = props => new PlaceholderSystemNote({
+      propsData: {
+        note: {
+          body: props,
+        },
+      },
+    }).$mount();
+  });
+
+  it('should render system note placeholder with plain text', () => {
+    const vm = mountComponent('This is a placeholder');
+
+    expect(vm.$el.tagName).toEqua('LI');
+
+    expect(vm.$el.querySelector('.timeline-content i').textContent.trim()).toEqua('This is a placeholder');
+  });
+});