libxml2: add meta.pkgConfigModules and tests.pkg-config

Co-authored-by: Rodney Lorrimar <[e-mail]>

pkgs/development/libraries/libxml2/default.nix

···
 , enableShared ? !stdenv.hostPlatform.isMinGW && !stdenv.hostPlatform.isStatic
 , enableStatic ? !enableShared
 , gnome
+, testers
 }:
 
 let
···
   assert oldVer -> stdenv.isDarwin; # reduce likelihood of using old libxml2 unintentionally
 
 let
-libxml = stdenv.mkDerivation rec {
+libxml = stdenv.mkDerivation (finalAttrs: rec {
   pname = "libxml2";
   version = "2.12.4";
 
···
       packageName = pname;
       versionPolicy = "none";
     };
+    tests = {
+      pkg-config = testers.hasPkgConfigModules {
+        package = finalAttrs.finalPackage;
+      };
+    };
   };
 
   meta = with lib; {
···
     license = licenses.mit;
     platforms = platforms.all;
     maintainers = with maintainers; [ eelco jtojnar ];
+    pkgConfigModules = [ "libxml-2.0" ];
   };
-};
+});
 in
 if oldVer then
   libxml.overrideAttrs (attrs: rec {

pkgs/top-level/all-packages.nix

···
   in pkgs.buildEnv { # slightly hacky
     name = "libxml2+py-${res.libxml2.version}";
     paths = with libxml2; [ dev bin py ];
-    # Avoid update.nix conflicts with libxml2.
-    passthru = builtins.removeAttrs libxml2.passthru [ "updateScript" ];
+    # Avoid update.nix/tests conflicts with libxml2.
+    passthru = builtins.removeAttrs libxml2.passthru [ "updateScript" "tests" ];
     # the hook to find catalogs is hidden by buildEnv
     postBuild = ''
       mkdir "$out/nix-support"