zlib: Add tests.pkg-configPackages

pkgs/development/libraries/zlib/default.nix

@@ -8,6 +8,7 @@
 # the `.pc` file lists only the main output's lib dir.
 # If false, and if `{ static = true; }`, the .a stays in the main output.
 , splitStaticOutput ? shared && static
+, tests
 }:
 
 # Without either the build will actually still succeed because the build
@@ -125,6 +126,14 @@ stdenv.mkDerivation rec {
     "SHARED_MODE=1"
   ];
 
+  passthru = {
+    tests = {
+      # NB: this may test a different variant of zlib than the package that
+      #     carries this `tests` attribute, due to overriding.
+      pkg-configPackages = tests.pkg-configPackages.zlib;
+    };
+  };
+
   meta = with lib; {
     homepage = "https://zlib.net";
     description = "Lossless data-compression library";