storage_tests: Fix another assertion output in postgresql tests

This fails in debian bullseye [1] preventing the current packaging and next deployment.

Reading the storage interface for that method, there is no order guarantee in the output.

This is a follow up on another mr [2] which was not enough to unstuck the packaging.

[1] https://jenkins.softwareheritage.org/view/swh-debian%20(draft)/job/debian/job/packages/job/DSTO/job/gbp-buildpackage/490/console

Refs. swh/infra/sysadm-environment#5047 (closed)

[2] !1085 (merged)