Fix flaky work items spec

What does this MR do and why?

This spec occasionally failed because it assumed the page was fully loaded, but often times it was blank. Fix this by looking for the CSS element that is needed in the test.

This seems to fail more often with Vite than Webpack.

Relates to #443737

How to set up and validate locally

Run bundle exec rspec './spec/features/projects/work_items/work_item_spec.rb[1:1:12:2:1]' many times.