nixos/test/hibernate: fix test

The test was failing since https://github.com/NixOS/nixpkgs/pull/488429
due to the conflicts property on the backdoor keeping it from being
restarted. From systemd's perspective both the stop and the start would
happen in the same transaction, and it doesn't like that.
This is fixed by instead shutting down the backdoor using a power-down
command.

nixos/tests/hibernate.nix

@@ -25,7 +25,7 @@ makeTest {
           ./common/auto-format-root-device.nix
         ];
 
-        systemd.services.backdoor.conflicts = [ "sleep.target" ];
+        powerManagement.powerDownCommands = "systemctl --no-block stop backdoor.service";
         powerManagement.resumeCommands = "systemctl --no-block restart backdoor.service";
 
         virtualisation.emptyDiskImages = [ (2 * config.virtualisation.memorySize) ];