adminsettings test: Fix save detection race condition

Use MutationObserver to detect if a saveProgress event was received,
which will trigger an animation.

Before this, `helper.admin$('#response').is(':visible')` was true
after the page loaded and before clicking the Save button, so there
was a possibility that after clicking Save, but before sending the
socketio message to the server, the visibility is checked and returns
true, so the page gets reloaded before the changed settings have been
saved.

src/tests/frontend/specs/adminsettings.js

@@ -21,6 +21,16 @@ describe('Admin > Settings', function () {
   });
 
   it('Are Settings visible, populated, does save work', async function () {
+    const save = async () => {
+      const p = new Promise((resolve) => {
+        const observer = new MutationObserver(() => { resolve(); observer.disconnect(); });
+        observer.observe(
+            helper.admin$('#response')[0], {attributes: true, childList: false, subtree: false});
+      });
+      helper.admin$('#saveSettings').click();
+      await p;
+    };
+
     // save old value
     const settings = helper.admin$('.settings').val();
     const settingsLength = settings.length;
@@ -29,10 +39,7 @@ describe('Admin > Settings', function () {
     helper.admin$('.settings').val((_, text) => `/* test */\n${text}`);
     await helper.waitForPromise(
         () => settingsLength + 11 === helper.admin$('.settings').val().length, 5000);
-
-    // saves
-    helper.admin$('#saveSettings').click();
-    await helper.waitForPromise(() => helper.admin$('#response').is(':visible'), 5000);
+    await save();
 
     // new value for settings.json should now be saved
     // reset it to the old value
@@ -44,9 +51,7 @@ describe('Admin > Settings', function () {
     // replace the test value with a line break
     helper.admin$('.settings').val((_, text) => text.replace('/* test */\n', ''));
     await helper.waitForPromise(() => settingsLength === helper.admin$('.settings').val().length);
-
-    helper.admin$('#saveSettings').click(); // saves
-    await helper.waitForPromise(() => helper.admin$('#response').is(':visible'));
+    await save();
 
     // settings should have the old value
     helper.newAdmin('settings');