test-instrumentation.nix: Set an empty root password

This makes it easier to log in during interactive sessions.

nixos/modules/testing/test-instrumentation.nix

@@ -97,6 +97,9 @@ let kernel = config.boot.kernelPackages.kernel; in
 
     networking.usePredictableInterfaceNames = false;
 
+    # Make it easy to log in as root when running the test interactively.
+    security.initialRootPassword = "";
+
   };
 
 }