Move prometheus_header specs to jest

app/assets/javascripts/monitoring/components/shared/prometheus_header.vue

@@ -10,6 +10,6 @@ export default {
 </script>
 <template>
   <div class="prometheus-graph-header">
-    <h5 class="prometheus-graph-title js-graph-title">{{ graphTitle }}</h5>
+    <h5 ref="title" class="prometheus-graph-title">{{ graphTitle }}</h5>
   </div>
 </template>

spec/javascripts/monitoring/shared/prometheus_header_spec.js → spec/frontend/monitoring/shared/prometheus_header_spec.js

@@ -18,7 +18,7 @@ describe('Prometheus Header component', () => {
 
   describe('Prometheus header component', () => {
     it('should show a title', () => {
-      const title = prometheusHeader.vm.$el.querySelector('.js-graph-title').textContent;
+      const title = prometheusHeader.find({ ref: 'title' }).text();
 
       expect(title).toBe('graph header');
     });