[test] Use store.prepare_insert_relation instead of deprecated relate method
Follow-up for 1f5026e7d848.
Related to #5040344.
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0