squashfsTools: add NixOS cdrom boot as passthru test

This is the same test which blocks nixos-unstable-small. It recently
caused a long blockage, due to a regression in squashfsTools itself
corrupting the iso image, see NixOS#132286.

pkgs/tools/filesystems/squashfs/default.nix

@@ -7,6 +7,7 @@
 , lz4
 , lzo
 , zstd
+, nixosTests
 }:
 
 stdenv.mkDerivation rec {
@@ -47,6 +48,10 @@ stdenv.mkDerivation rec {
     "LZO_SUPPORT=1"
   ];
 
+  passthru.tests = {
+    nixos-iso-boots-and-verifies = nixosTests.boot.biosCdrom;
+  };
+
   meta = with lib; {
     homepage = "https://github.com/plougher/squashfs-tools";
     description = "Tool for creating and unpacking squashfs filesystems";