plug-ins: add file-plug-in testing framework

This adds file plug-in testing to our CI for a large part of our supported file formats. Currently only import testing is enabled, though a start was made on implementing export testing.

Besides batch import and export testing, this testing can also be done from within GIMP (listed under Filters -> Development -> Python-Fu), provided some env vars have been set up to point to the location of the test images, log file, and main config file. Details about that are present in the readme. The plug-ins are only installed on unstable builds.

@brunolopesdsilv Since you are the expert on .gitlab-ci.yml maybe you could check if there is anything to be improved there.

@Jehan, @cmyk.student Any thoughts on how the testing is currently done, otherwise I'll probably merge soon. It has already caught some issues. Eventually we need to extend it to test all file formats we support.