fix test error with node 14

This worked fine on my workstation with node 12. But on CI or my laptop
with node 14, it failed. Apparently there @testing-library/user-events
expects a peer @testing-library/dom dependency. At least copying the
"npm install --save-dev @testing-library/user-event @testing-library/dom" command
from the top of https://testing-library.com/docs/ecosystem-user-event/
made it work again on my laptop; fingers crossed about CI.

ui/package.json

@@ -64,6 +64,7 @@
     ]
   },
   "devDependencies": {
+    "@testing-library/dom": "^8.1.0",
     "@testing-library/jest-dom": "^5.11.4",
     "@testing-library/react": "^11.2.7",
     "@testing-library/user-event": "^12.8.3",