test: restore ns_last_pid before executing restore in a second time

It is required for cases when we inject a fault in criu restore.
In this case we execute "criu restore" and check that it fails,
then we execute "criu restore" without a fault and check that it passes.

If the first "criu restore" restores only a part of processes,
the second criu can get PID of one of restored processes.

https://github.com/xemul/criu/issues/282


Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>