Add an option to write the X session log to the journal

... rather than ~/.xsession-errors. It might make sense to make this
the default, in order to eliminate ad hoc, uncentralised, poorly
discoverable log files.

nixos/modules/testing/test-instrumentation.nix

@@ -113,6 +113,8 @@ let kernel = config.boot.kernelPackages.kernel; in
     # Make it easy to log in as root when running the test interactively.
     users.extraUsers.root.initialHashedPassword = mkOverride 150 "";
 
+    services.xserver.displayManager.logToJournal = true;
+
   };
 
 }