storage_tests: Fix other assertions in postgresql tests

This times i chased remaining directory_missing calls to replace them with sets.

This still fails in debian unstable [1]

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

Follow up from:

• !1090 (merged)
• !1085 (merged)

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

[1] https://jenkins.softwareheritage.org/view/swh-debian%20(draft)/job/debian/job/packages/job/DSTO/job/gbp-buildpackage/491/console