Setup integration test setup

- Also moves special snapshot serializer to its
  own module.

spec/frontend_integration/test_helpers/setup/index.js

+import '../../../frontend/test_setup';
+import './setup_globals';
+import './setup_axios';
+import './setup_serializers';

spec/frontend_integration/test_helpers/setup/setup_axios.js

+import axios from '~/lib/utils/axios_utils';
+import adapter from 'axios/lib/adapters/xhr';
+
+// We're removing our default axios adapter because this is handled by our mock server now
+axios.defaults.adapter = adapter;

spec/frontend_integration/test_helpers/setup/setup_globals.js

+import { setTestTimeout } from 'helpers/timeout';
+
+beforeEach(() => {
+  window.gon = {
+    api_version: 'v4',
+    relative_url_root: '',
+  };
+
+  setTestTimeout(5000);
+  jest.useRealTimers();
+});
+
+afterEach(() => {
+  jest.useFakeTimers();
+});

spec/frontend_integration/test_helpers/setup/setup_serializers.js

+import defaultSerializer from '../snapshot_serializer';
+
+expect.addSnapshotSerializer(defaultSerializer);

spec/frontend_integration/test_helpers/snapshot_serializer.js

+export default {
+  test(value) {
+    return value instanceof HTMLElement && !value.$_hit;
+  },
+  print(element, serialize) {
+    element.$_hit = true;
+    element.querySelectorAll('[style]').forEach(el => {
+      el.$_hit = true;
+      if (el.style.display === 'none') {
+        el.textContent = '(jest: contents hidden)';
+      }
+    });
+
+    return serialize(element)
+      .replace(/^\s*<!---->$/gm, '')
+      .replace(/\n\s*\n/gm, '\n');
+  },
+};

spec/frontend_integration/test_setup.js

+import './test_helpers/setup';