Mark a subgroup-related spec as PostgreSQL-only

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/34847 to resolve failing master.

Also reorders the variables so they're in logical order and removes the unnecessary let! in favor of let.

approved this merge request

enabled an automatic merge when the pipeline for 09aec212 succeeds

mentioned in issue #34847 (closed)

Though I just realized my unnecessary variable re-ordering will probably create an EE conflict.

mentioned in merge request !12723 (closed)

merged

mentioned in commit 5b257778