Fix incorrect UpgradeController test

The current version is set to 2.3 at the top level of the test. If the
test case uses 5.15.1 as the latest version, then the test should've
failed – but it didn't because we're missing a few promises to be
awaited.

To verify, try reverting the version diff and re-run the test. It would
fail with the new assertions.

client/src/controllers/UpgradeController.test.js

@@ -8,7 +8,7 @@ describe('UpgradeController', () => {
   const version = '2.3';
 
   beforeEach(() => {
-    document.body.innerHTML = `
+    document.body.innerHTML = /* html */ `
     <div
       class="panel"
       id="panel"
@@ -78,7 +78,7 @@ describe('UpgradeController', () => {
 
   it('should not show the message if the current version is up to date', async () => {
     const data = {
-      version: '5.15.1',
+      version: '2.3',
       url: 'https://docs.wagtail.org/latest/url',
       minorUrl: 'https://docs.wagtail.org/latest-minor/url',
       lts: {
@@ -99,6 +99,16 @@ describe('UpgradeController', () => {
     // trigger next browser render cycle
     await Promise.resolve();
 
+    expect(global.fetch).toHaveBeenCalledWith(
+      'https://releases.wagtail.org/mock.txt',
+      { referrerPolicy: 'strict-origin-when-cross-origin' },
+    );
+
+    expect(document.getElementById('panel').hidden).toBe(true);
+
+    await new Promise(requestAnimationFrame);
+
+    // should keep the hidden attribute
     expect(document.getElementById('panel').hidden).toBe(true);
   });