[tox] Split migractions tests from server environment These are the longest tests in this environment, so running them apart (in parallel of others) would hopefully speed up the whole suite.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/python2*/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl