nixosTests.custom-ca: resolve out of memory situations

They are easily observable on hydra. E.g. on the latest eval

https://hydra.nixos.org/build/233893887
https://hydra.nixos.org/build/233900101

nixos/tests/custom-ca.nix

@@ -131,8 +131,8 @@ let
         # chromium-based browsers refuse to run as root
         test-support.displayManager.auto.user = "alice";
 
-        # browsers may hang with the default memory
-        virtualisation.memorySize = 600;
+        # machine often runs out of memory with less
+        virtualisation.memorySize = 1024;
 
         environment.systemPackages = [ pkgs.xdotool pkgs.${browser} ];
       };