PLCgen: incorporate self-certification tests as integration tests

We made a self-certification test set for PLCgen in #798 (closed). They are available as ZIP files and a branch. Since we now have a working version of PLCgen, it is time to integrate the self-certification test set into the integration tests of PLCgen. I propose to have two folders: one for the certification set, and one for the existing tests, which are then maybe a 'generator' test set or so. We can remove from the 'generator' part the things that are already covered by the certification set.

• Add certification tests. (!1180 (merged))
• Remove duplicates from 'generator' tests.
• Update 'copy_all' script for having sub-directories in PLCgen tests. (!1183 (merged))

Addresses #679