additional asserts for test backup

[pondrejk]

related to #4
sample test

pytest --ansible-host-pattern satellite --ansible-user=root  --ansible-inventory testfm/inventory tests/test_backup.py::test_positive_backup_online_skip_pulp_content 
============================================= test session starts ==============================================
platform linux2 -- Python 2.7.15, pytest-3.4.0, py-1.5.3, pluggy-0.6.0
ansible: 2.3.2.0
rootdir: /home/pondrejk/Documents/testfm, inifile:
plugins: ansible-1.3.1
collected 1 item                                                                                               

tests/test_backup.py .                                                                                   [100%]

========================================== 1 passed in 88.42 seconds ===========================================

[ntkathole]

ACK