tests/emacs-daemon: fix failure for unset $DISPLAY

axelf4 figured[1] out a workaround for fixing a failure due to an unset
$DISPLAY variable.

[1] https://github.com/NixOS/nixpkgs/pull/168076#issuecomment-1098083299

nixos/tests/emacs-daemon.nix

@@ -33,7 +33,7 @@ import ./make-test-python.nix ({ pkgs, ...} : {
       )
 
       # connects to the daemon
-      machine.succeed("emacsclient --create-frame $EDITOR >&2 &")
+      machine.succeed("emacsclient --no-wait --frame-parameters='((display . \"'\"$DISPLAY\"'\"))' --create-frame $EDITOR >&2")
 
       # checks that Emacs shows the edited filename
       machine.wait_for_text("emacseditor")