Buildbot: Save entire var/ contents when test fails

In order to help debug race conditions that happen sporadically, we need the contents of var/ directory to be saved.

As this folder can be rather big, save it only when the run has failed. We can discuss smarter approaches later if space is a big issue.

Identify a time frame for how long these files need to be stored.

Comments:
Note that tests are now run with --max-save-core=1 --max-save-datadir=1
Saving one core is probably good enough. Having even one failed test directory is much better than nothing, but it would be good to have at least 3 or 4. This will of course put some constraints on shared memory, which needs to be considered.
Time from of 48 hours should be enough, or even better a round robin of as many tests that fits into a given size, like 10G or so.