Unittest for backup.py with coverage >= 90%

More Unittest needs to be implemented in Freezer.
The total coverage have to be at least 90%.
This commit is for freezer.backup unittest.

setup.py and tests directory re organized
according pytest best practices.

Change-Id: I61aa1b58d9501b81fda37dd785add27ba86d01c9
LAUNCHPAD: https://blueprints.launchpad.net/freezer/+spec/test-backup.py