nixosTests.fontconfig-default-fonts: use Gentium font

As GentiumPlus will probably never be updated further by upstream.

nixos/tests/fontconfig-default-fonts.nix

···
         cantarell-fonts
         twitter-color-emoji
         source-code-pro
-        gentium-plus
+        gentium
       ];
       fonts.fontconfig.defaultFonts = {
-        serif = [ "Gentium Plus" ];
+        serif = [ "Gentium" ];
         sansSerif = [ "Cantarell" ];
         monospace = [ "Source Code Pro" ];
         emoji = [ "Twitter Color Emoji" ];
···
     };
 
   testScript = ''
-    machine.succeed("fc-match serif | grep '\"Gentium Plus\"'")
+    machine.succeed("fc-match serif | grep '\"Gentium\"'")
     machine.succeed("fc-match sans-serif | grep '\"Cantarell\"'")
     machine.succeed("fc-match monospace | grep '\"Source Code Pro\"'")
     machine.succeed("fc-match emoji | grep '\"Twitter Color Emoji\"'")