* Avoid test failure by not checking exact size of backup
This is a workaround for the fact that the backup size is not exactly
the same every time. This is due to the fact that the inner gziped tar
file can vary in size due to difference in json file (key order) and
potentially also different field values (UUID, backup slug).
It seems that sorting the keys makes the actual difference today, but
this has runtime overhead and might not catch all cases.
Simply check if size property is there and a number bigger than 0
instead.
* Fix pytest