Better testcase snapshots
Implementing improvements discussed in https://lab.nexedi.com/nexedi/slapos/commit/1a5df533809238a82b09fc9aae56bc90e31672af#note_93275 and on https://lab.nexedi.com/nexedi/slapos.core/merge_requests/150#note_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.