Fix some test wait conditions

ui-tests/test/general.spec.ts

@@ -23,6 +23,9 @@ test.describe('General', () => {
       ".jp-Notebook-ExecutionIndicator[data-status='idle']"
     );
 
+    // wait for the checkpoint indicator to be displayed
+    await page.waitForSelector('.jp-NotebookCheckpoint');
+
     // force switching back to command mode to avoid capturing the cursor in the screenshot
     await page.evaluate(async () => {
       await window.jupyterapp.commands.execute('notebook:enter-command-mode');

ui-tests/test/notebook.spec.ts

@@ -7,7 +7,7 @@ import { expect } from '@playwright/test';
 
 import { test } from './fixtures';
 
-import { runAndAdvance, waitForKernelReady } from './utils';
+import { runAndAdvance } from './utils';
 
 const NOTEBOOK = 'example.ipynb';
 
@@ -70,8 +70,6 @@ test.describe('Notebook', () => {
     );
     await page.goto(`notebooks/${tmpPath}/${notebook}`);
 
-    await waitForKernelReady(page);
-
     // execute the first cell
     await runAndAdvance(page);
     await page