testing/testcase: separate instance/software snapshot

Log files from slapos node software are large and they are always the
same after each test method, so it does not make much sense to store
them for each test method snapshot, because we only run slapos node
software at setupModule step.

Log files from slapos node instance are smaller, but it's also not
needed to make a snapshot after each test, because we only run slapos
node instance at setUpClass step.

To prevent duplication, store slapos log files only at setupModule (for
software) and at setUpClass (for instance).

Also store log slapos log files in all the steps that can fail in
_cleanup.