Add CI test for this very repo

[htgoebel]

When I tried to reproduce #53 only four month later, this failed. I suggest adding CI testing for this very repo - following the instructions from the readme - to detect issues due to e.g. changes to the exteneds files and to ensure one can actually set to a testing environment for the respective Plone versions.

[idgserpro]

@htgoebel I was able to reproduce the restart error using the steps described by you at:

#53 (comment)