[web/test] don't depend on iteration order on a set
Fixes test under python -R ('pending_insert' in session data is a set,
so we can't rely on any particular ordering). Related to #4959402.
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