fix core/tests/test_population.py so it can be run from any path
fix core/tests/test_population.py so it can be run from any path

lidakanari/NeuroM,juanchopanza/NeuroM,eleftherioszisis/NeuroM,wizmer/NeuroM,mgeplf/NeuroM,liesbethvanherpe/NeuroM,BlueBrain/NeuroM