[test] Drop call to CubicWebConfiguration.load_cwctl_plugins in unittest_cwctl.py This is no longer need now that we call load_available_configurations() in the "list" command (see 5e7282bdf140).

pytest
http://hg.logilab.org/master/logilab/common/archive/default.tar.bz2#egg=logilab-common
http://hg.logilab.org/master/yams/archive/default.tar.bz2#egg=yams