Adding Visual Expectations for Plotting Functions

[mariusbarth]

Hi @crsh,

It is still one of my aims to refactor the family of apa_*plot() functions in the future. Actually, I already drafted a powerful approach, but the changes are so extensive that it is practically impossible to review these changes all at once and I also want to take some precautions not to break older code.

Therefore, I thought it is wise to first add visual expectations (via the vdiffr package) before I start adding theses changes one by one.

This PR is intended to get github actions running, and to gain some experience with vdiffr. I will require your review as soon as I have the impression that we can add these tests to the main branch. In the meantime, any feedback on how to improve the testing pipeline is highly appreciated.