executor: cgroups setup causes unrecoverable errors

[dvyukov]

I don't have full proof, but looking at spikes of SYZFATAL errors I think the following can happen.
We create cgroups of the for "/syzcgroup/*/syz{{PROCID}}" and then use this cgroup for all tests for PROCID, we don't create per-test cgroups. So if a test messes with cgroup settings, e.g. restricts memory to a very low value, all subsequent tests for the same PROCID will fail. Even if executor is restarted.
We either should create per-test cgroups, or at least per loop process cgropus, so that restarting executor fixes things.