CI: Running tests with xvfb for the clipboard to work
- [X] closes #26887
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

With some luck this should make the clipboard tests work in azure. @jorisvandenbossche @jreback can you have a look once the CI finishes?