nixos/qemu-vm: default memorySize 384 -> 1024

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

nixos/tests/vault-postgresql.nix

@@ -12,7 +12,6 @@ import ./make-test-python.nix ({ pkgs, ... }:
     maintainers = [ lnl7 roberth ];
   };
   machine = { lib, pkgs, ... }: {
-    virtualisation.memorySize = 512;
     environment.systemPackages = [ pkgs.vault ];
     environment.variables.VAULT_ADDR = "http://127.0.0.1:8200";
     services.vault.enable = true;