Add explicit video retain-on-failure

Author: jtpio

ui-tests/playwright.config.ts

@@ -3,7 +3,8 @@ import baseConfig from '@jupyterlab/galata/lib/playwright-config';
 module.exports = {
   ...baseConfig,
   use: {
-    appPath: ''
+    appPath: '',
+    video: 'retain-on-failure'
   },
   retries: 1
 };