Test GDM in gnome3-xorg test

[worldofpeace]

Describe the bug

Currently with the following diff

diff --git a/nixos/tests/gnome3-xorg.nix b/nixos/tests/gnome3-xorg.nix
index f12361da037..bfc76c39345 100644
--- a/nixos/tests/gnome3-xorg.nix
+++ b/nixos/tests/gnome3-xorg.nix
@@ -11,13 +11,18 @@ import ./make-test.nix ({ pkgs, ...} : {
 
       services.xserver.enable = true;
 
-      services.xserver.displayManager.gdm.enable = false;
-      services.xserver.displayManager.lightdm.enable = true;
-      services.xserver.displayManager.lightdm.autoLogin.enable = true;
-      services.xserver.displayManager.lightdm.autoLogin.user = "alice";
       services.xserver.desktopManager.gnome3.enable = true;
       services.xserver.desktopManager.default = "gnome-xorg";
 
+      services.xserver.displayManager.gdm = {
+        wayland = false;
+        enable = true;
+        autoLogin = {
+          enable = true;
+          user = "alice";
+        };
+      };
+
       virtualisation.memorySize = 1024;
     };
 

it stalls with this repeating message in the output

machine: must succeed: xwininfo -root -tree | sed 's/.*0x[0-9a-f]* \"\([^\"]*\)\".*/\1/; t; d'
machine# No protocol specified
machine# xwininfo: error: unable to open display ":0.0"
machine: exit status 0
(0.04 seconds)
machine: must succeed: xwininfo -root -tree | sed 's/.*0x[0-9a-f]* \"\([^\"]*\)\".*/\1/; t; d'
machine# No protocol specified
machine# xwininfo: error: unable to open display ":0.0"
machine: exit status 0
(0.03 seconds)

cc @jtojnar

[worldofpeace]

I could add something like

setXSession = session: "${gdbus} call --system -d org.freedesktop.Accounts -o /org/freedesktop/Accounts/User1000 -m org.freedesktop.Accounts.User.SetXSession ${session}";

services.xserver.displayManager.job.preStart = setXSession "gnome-flashback-${wmName}";

for the flashback tests and probably rewrite the gnome3-xorg one. But I'm not sure if it's worth it just yet if there's development happening on the python testing driver.
(this would be some functionality I'd want within it...)

[worldofpeace]

Yay, we rewrote the test and with the help of https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/x11/display-managers/set-session.py allows us to force the session to gnome-xorg.