Catches failures like https://github.com/NixOS/nixpkgs/issues/149539
that don't happen with AutomaticLoginEnable.
We still have a 0-delay autologin test in gnome-xorg, in case there's
ever an issue that only arises with AutomaticLoginEnable.
the default hasn't been changed since 2009
this can improve our test performances
nixos/tests: remove explicit memorySize <1024
1024MiB is now the default