NixOS tests: Increase hung_task_timeout_secs

This hopefully fixes intermittent test failures like

  http://hydra.nixos.org/build/29962437

  router# [  240.128835] INFO: task mke2fs:99 blocked for more than 120 seconds.
  router# [  240.130135]       Not tainted 3.18.25 #1-NixOS
  router# [  240.131110] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.

assuming that these are caused by high load on the host.