Tests: add test for the removal of tags
Tests: add test for the removal of tags

This test makes sure the actual tag class instance is also removed after
the last link to a tag is removed.

htem/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID