[web/test] use unicode object as user login
(py.warnings) WARNING: /home/jcr/src/cw/cubicweb/devtools/testlib.py:206: DeprecationWarning: Sanitizing an input dictionary with str values, please check your data (key = 'login')
usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py