php: fix build on darwin (the iconv extension)

https://hydra.nixos.org/build/258946784/nixlog/2/tail
The issue was probably triggered by the darwin iconv changes
done during this staging-next cycle.

Also use enable tests on linux, as they pass for me
on native x86_64 and aarch64.

pkgs/top-level/php-packages.nix

@@ -428,10 +428,9 @@ in {
         }
         {
           name = "iconv";
-          configureFlags = [
-            "--with-iconv${lib.optionalString stdenv.isDarwin "=${libiconv}"}"
-          ];
-          doCheck = false;
+          buildInputs = [ libiconv ];
+          configureFlags = [ "--with-iconv" ];
+          doCheck = stdenv.isLinux;
         }
         {
           name = "imap";