[server/test] make unittest_migractions.py use new style cubes
MigrationTC used old cube layout in
cubicweb/server/test/data-migractions/cubes, move them to new style cubes as
"cubicweb_<cubename>" in "apphome" (cubicweb/server/test/data-migractions/)
which is added to PYTHONPATH by CubicWebTC.