grsecurity test: verify that the grsec device node is created

nixos/tests/grsecurity.nix

···
   machine = { config, pkgs, ... }:
     { security.grsecurity.enable = true;
       boot.kernel.sysctl."kernel.grsecurity.deter_bruteforce" = 0;
-      security.apparmor.enable = true;
     };
 
   testScript = ''
···
     subtest "tcc", sub {
       $machine->execute("echo -e '#include <stdio.h>\nint main(void) { puts(\"hello\"); return 0; }' >main.c");
       $machine->succeed("${pkgs.tinycc.bin}/bin/tcc -run main.c");
+    };
+
+    subtest "RBAC", sub {
+      $machine->succeed("[ -c /dev/grsec ]");
     };
   '';
 })