Improved handling of users/groups captured during backup that do not exist on the restore host. Also explicitly handle the case where user/group is not mapped to a name.
This was worked out as part of the test suite refactor [c8f806a] but not committed with it because of the large number of expect logs changes involved. Keeping them separate made it easier to audit the changes in the refactor.
* Make the code more modular and object-oriented.
* Multiple Docker containers can now be created for a single test to simulate more realistic environments.
