Fix test_remove_backups() for all test cases and code cleanup
Fix test_remove_backups() for all test cases and code cleanup

  Rather than checking the length of os.listdir() in test_remove_backups(),
  actually check the content returned by list_dest_backups() (and os.listdir()
  separately when appropriate) this needs to be done to check for proper
  rotation.

jesseelwell/python-backup