test: refactor gh_6305_net_box_autocomplete

Test is needed to be updated every time net.box's internals are patched
(e.g. in case of adding new functions of deleting old ones), as the order
of suggested autocomplete options changes.

Let's replace `assert_equals` with `assert_items_equals`, which is not
order dependent.

NO_DOC=test
NO_CHANGELOG=test