Refactor backup manager specs

What does this MR do and why?

Part of #351383 (closed)

Fixes #352907 (closed)

This is continuing work started in !79809 (merged) where all the backup task logic was moved into the manager so that logic is centralised and state can be shared easier.

• Extract pre/post warning from the DB task so that asking the user for input is centralised and can be tested easier. This stops the DB task from being so much of a special case.
• Extract task definitions. Since backup.rake still has integration tests making sure backups on the whole work, we would like to be able to rewrite the tests just to test packing and task running alone. This should allow faster and more focused testing.
• Refactor tests into new restricted public interface. backup.rake is the only user of Manager and it now only uses a handful of methods, anything else should be private. This requires most of the tests to be adjusted. This should allow easier internal refactoring.

How to set up and validate locally

1. Create a backup.

   $ bundle exec rake gitlab:backup:create
   ...
   Creating backup archive: 1644531746_2022_02_11_14.8.0-pre_gitlab_backup.tar ... done
   Uploading backup archive to remote storage  ... skipped
   Deleting tmp directories ... done
   done
   done
   done
   done
   done
   done
   done
   done
   done
   Deleting old backups ... skipping
   Warning: Your gitlab.rb and gitlab-secrets.json files contain sensitive data 
   and are not included in this backup. You will need these files to restore a backup.
   Please back them up manually.
   Backup task is done.

2. Restore from the backup. Note that BACKUP does not take a full path.

   $ bundle exec rake gitlab:backup:restore BACKUP=1644531746_2022_02_11_14.8.0-pre
   ...

MR acceptance checklist

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