test/sandbox: verify seccomp on all test cases

This change also makes seccomp hashes cross-platform.

Signed-off-by: Ophestra <cat@gensokyo.uk>

test/sandbox/configuration.nix

@@ -75,6 +75,6 @@ in
       }
     ];
 
-    apps = import ./case lib testProgram;
+    apps = import ./case pkgs.system lib testProgram;
   };
 }