Headless or Mock backend for running tests inside CI

[aminya]

Feature category

• [ ] enhancement - build system
• [x] enhancement - backends
• [ ] enhancement - build system
• [ ] enhancement - documentation
• [ ] enhancement - plot categories

The problem It would be nice if there was a headless or mock backend that could be used inside CI for the tests.

The solution I'd like

This backend would use the matplotplusplus logic, but will not create a real plot.

Alternatives I've considered

Currently, I have to disable the plots to test my code inside CI.

Additional context