nixos/tests/hibernate: disable for platforms other than x86_64

Due to 9pnet_virtio bugs, /nix is no longer available after
hibernation. It happens to work on x86_64, but not on other
platforms.

nixos/tests/all-tests.nix

@@ -120,7 +120,10 @@ in
   handbrake = handleTestOn ["x86_64-linux"] ./handbrake.nix {};
   haproxy = handleTest ./haproxy.nix {};
   hardened = handleTest ./hardened.nix {};
-  hibernate = handleTest ./hibernate.nix {};
+  # 9pnet_virtio used to mount /nix partition doesn't support
+  # hibernation. This test happens to work on x86_64-linux but
+  # not on other platforms.
+  hibernate = handleTestOn ["x86_64-linux"] ./hibernate.nix {};
   hitch = handleTest ./hitch {};
   hocker-fetchdocker = handleTest ./hocker-fetchdocker {};
   home-assistant = handleTest ./home-assistant.nix {};