We need to move NixOS containers somewhere else so these don't clash
with Podman, Skopeo & other container software in the libpod &
cri-o/cri-u/libcontainer ecosystems.
The state directory move is not strictly a requirement but is good for
consistency.
the default hasn't been changed since 2009
this can improve our test performances
nixos/tests: remove explicit memorySize <1024
1024MiB is now the default
