Missing spec coverage for testing detached partitions - spec/lib/gitlab/database/tables_locker_spec.rb

See !111045 (comment 1278531344)

If we comment out Postgresql::DetachedPartition.find_each in lib/gitlab/database/tables_locker.rb, it seems spec/lib/gitlab/database/tables_locker_spec.rb still passes

/cc @rutgerwessels @OmarQunsulGitlab