Use pytest-xvfb

Use pytes-xvfb for tests.

This provides a better xvfb runner, ensuring the service is working before using it.

Few arguments are provided to pytest, like --no-xvfb (to use your screen, instead of a virtual one)

Extra dependencies:

• https://github.com/The-Compiler/pytest-xvfb
• https://github.com/ponty/PyVirtualDisplay/blob/master/pyvirtualdisplay/abstractdisplay.py

I expect it to close #2597 (closed)