Each test module setups the path to find the needed modules, but didn't check if it was necesary. Creating a long path that finally started to fail.
Implemented in a different way. For all internal commands now we use Python bindings to fill the zones as suggested by @arikrupnik in johnbeard/kiplot#11 For 'print_pcb' now we ask KiCad to do it before printing (needs v1.4.1)