gtk: fix missing newline in formatted config (#2809)

The conversion from `concatMapStrings` to `concatStringsSep` introduced in https://github.com/nix-community/home-manager/pull/2481
creates an unintended behavior change where the formatted config does not end in a newline.[1]
This is problematic for manipulation at the Nix level. In particular, this cause a regression in
the generation of gtk2 settings due to concatenated of the formatted config and `gtk2.extraConfig`
without a newline in between.

This commit restores `concatMapStrings` to match the previous behavior and adds a newline to
the final string for the generated gtk2 config. The test case for gtk2-basic-config
was also updated to check behavior at concatenation boundaries.

[1] - https://github.com/nix-community/home-manager/pull/2481#discussion_r830648706

modules/misc/gtk.nix

@@ -250,8 +250,8 @@ in {
     ];
 
     home.file.${cfg2.configLocation}.text =
-      concatStringsSep "\n" (mapAttrsToList formatGtk2Option gtkIni)
-      + cfg2.extraConfig;
+      concatMapStrings (l: l + "\n") (mapAttrsToList formatGtk2Option gtkIni)
+      + cfg2.extraConfig + "\n";
 
     home.sessionVariables.GTK2_RC_FILES = cfg2.configLocation;
 
@@ -262,7 +262,7 @@ in {
       mkIf (cfg3.extraCss != "") { text = cfg3.extraCss; };
 
     xdg.configFile."gtk-3.0/bookmarks" = mkIf (cfg3.bookmarks != [ ]) {
-      text = concatStringsSep "\n" cfg3.bookmarks;
+      text = concatMapStrings (l: l + "\n") cfg3.bookmarks;
     };
 
     xdg.configFile."gtk-4.0/settings.ini".text =

tests/modules/misc/gtk/gtk2-basic-config-expected.conf

@@ -1 +1,2 @@
+gtk-theme-name = "Adwaita"
 gtk-can-change-accels = 1

tests/modules/misc/gtk/gtk2-basic-config.nix

@@ -6,6 +6,7 @@ with lib;
   config = {
     gtk = {
       enable = true;
+      theme.name = "Adwaita";
       gtk2.extraConfig = "gtk-can-change-accels = 1";
     };