[test] Use store.prepare_insert_relation instead of deprecated relate method
Follow-up for 1f5026e7d848.
Related to #5040344.
/*
* cubicweb js mock for unit tests
* This module defines variables and functions used in quite a few places
* in cw js framework that can't be used or guessed without a real CW server
*/
var pageid = 'my-page-id';
function _(message) {
return message;
}