fix test_regions: test only existing cell and vertex groups
fix test_regions: test only existing cell and vertex groups

sfepy/sfepy,BubuLK/sfepy,sfepy/sfepy,rc/sfepy,vlukes/sfepy,BubuLK/sfepy,BubuLK/sfepy,vlukes/sfepy,vlukes/sfepy,rc/sfepy,sfepy/sfepy,rc/sfepy