Better testcase snapshots

Implementing improvements discussed in slapos@1a5df533 (comment 93275) and on  !150 (comment 92471) 

We now only store slapos logs one per `setUpModule` for software and `setupClass` for instances.

There's no de-duplication, but it's still a bit big.

/reviewed-on !156
Status Job ID Name Coverage
  External
failed SlapOS.Eggs.UnitTest-Master.Python2

00:17:57

failed SlapOS.Eggs.UnitTest-Master.Python3

00:17:39

failed SlapOS.Eggs.UnitTest-Master.Python2

00:13:10

failed SlapOS.Eggs.UnitTest-Master.Python2

00:13:26

failed SlapOS.Eggs.UnitTest-Master.Python2

00:14:08

failed SlapOS.Eggs.UnitTest-Master.Python2

00:18:21

failed SlapOS.Eggs.UnitTest-Master.Python3

00:15:51

failed SlapOS.Eggs.UnitTest-Master.Python3

00:13:42

failed SlapOS.Eggs.UnitTest-Master.Python3

00:14:49

failed SlapOS.Eggs.UnitTest-Master.Python3

00:13:41