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.

nixos/modules/system/boot/stage-1-init.sh

@@ -140,6 +140,7 @@ done
 
 
 # Create device nodes in /dev.
+@preDeviceCommands@
 echo "running udev..."
 mkdir -p /etc/udev
 ln -sfn @udevRules@ /etc/udev/rules.d

nixos/modules/system/boot/stage-1.nix

@@ -203,7 +203,7 @@ let
     inherit (config.boot) resumeDevice devSize runSize;
 
     inherit (config.boot.initrd) checkJournalingFS
-      postEarlyDeviceCommands preLVMCommands postDeviceCommands postMountCommands kernelModules;
+      preLVMCommands preDeviceCommands postEarlyDeviceCommands postDeviceCommands postMountCommands kernelModules;
 
     resumeDevices = map (sd: if sd ? device then sd.device else "/dev/disk/by-label/${sd.label}")
                     (filter (sd: (sd ? label || hasPrefix "/dev/" sd.device) && !sd.randomEncryption) config.swapDevices);
@@ -303,6 +303,15 @@ in
       '';
     };
 
+    boot.initrd.preDeviceCommands = mkOption {
+      default = "";
+      type = types.lines;
+      description = ''
+        Shell commands to be executed before udev is started to create
+        device nodes.
+      '';
+    };
+
     boot.initrd.postDeviceCommands = mkOption {
       default = "";
       type = types.lines;

nixos/modules/testing/test-instrumentation.nix

@@ -43,6 +43,11 @@ let kernel = config.boot.kernelPackages.kernel; in
     # into thinking they're running interactively.
     environment.variables.PAGER = "";
 
+    boot.initrd.preDeviceCommands =
+      ''
+        echo 600 > /proc/sys/kernel/hung_task_timeout_secs
+      '';
+
     boot.initrd.postDeviceCommands =
       ''
         # Using acpi_pm as a clock source causes the guest clock to