This commit updates `S3Inventory#files` to ignore S3 inventory files
which have a `last_modified` timestamp which are not at least 2 days
older than `BackupMetadata.last_restore_date` timestamp.
This check was previously only in `Jobs::EnsureS3UploadsExistence` but
`S3Inventory` can also be used via Rake tasks so this protection needs
to be in `S3Inventory` and not in the scheduled job.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.