catch2_3: add spdlog to passthru.tests

see https://github.com/NixOS/nixpkgs/pull/435366#issuecomment-3255581683

pkgs/by-name/ca/catch2_3/package.nix

@@ -4,6 +4,7 @@
   fetchFromGitHub,
   cmake,
   python3,
+  spdlog,
 }:
 
 stdenv.mkDerivation rec {
@@ -55,6 +56,10 @@ stdenv.mkDerivation rec {
     python3
   ];
 
+  passthru.tests = {
+    inherit spdlog;
+  };
+
   meta = {
     description = "Modern, C++-native, test framework for unit-tests";
     homepage = "https://github.com/catchorg/Catch2";