[web/test] don't delete uicache directory
Other tests use it, so the rmtree was a bit too much. Set CACHEDIR to a
temporary subdirectory, and only delete that.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/