Intermittent failures in traitsui.tests.editors.test_html_editor.py

Observed in #1546

From the CI logs:
```
test_base_url_changed (traitsui.tests.editors.test_html_editor.TestHTMLEditor) ... Command '['edm', 'run', '-e', 'traitsui-test-3.6-pyside2', '--', 'python', '-W', 'default', '-m', 'coverage', 'run', '-p', '-m', 'unittest', 'discover', '-v', 'traitsui']' returned non-zero exit status 252.
/usr/bin/bash /home/runner/work/_actions/GabrielBB/xvfb-action/v1/cleanup.sh
Killing the following xvfb processes: 5529
Error: The process '/usr/bin/xvfb-run' failed with exit code 1
```

after enabling faulthandler in tests this failure was no longer observed. I was unable to reproduce failure locally on MacOS or Ubuntu (failure was occurring in Ubuntu/Pyside2 CI run).

I apologize for the vague issue, I do not currently have a good understanding of what occurred or how to easily reproduce it, but didn't want it to go untracked entirely.