Disambiguate 'when local file is used' contexts in uploaders/object_storage_spec.rb

This issue came out of a discussion https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/314/diffs#note_302123646.

uploaders/object_storage_spec.rb has two context blocks on the same tree level that are both labelled 'when local file is used':

1. https://gitlab.com/gitlab-org/gitlab/-/blob/44002f46888a5944c710a04a016f6fa3ef45e466/spec/uploaders/object_storage_spec.rb#L599
2. https://gitlab.com/gitlab-org/gitlab/-/blob/44002f46888a5944c710a04a016f6fa3ef45e466/spec/uploaders/object_storage_spec.rb#L615

This issue is to decide if:

• one, or both, should be renamed to disambiguate them. Or:
• they can be merged