bottom: fix empty-settings test on darwin (#2603)

The empty configuration test for the bottom module introduced as of https://github.com/nix-community/home-manager/pull/2323
is not cross platform. Specifically, it silently fails under a darwin environment due to
the configuration file not being generated at $XDG_CONFIG_HOME. This PR add cross platform support
by specifying the platform-dependent configuration directories to check. The expected unit test data
was also extracted to a separate file to differentiate between test data changes and
changes to the test itself.

tests/modules/programs/bottom/empty-settings.nix

@@ -9,8 +9,13 @@ with lib;
       package = config.lib.test.mkStubPackage { };
     };
 
-    nmt.script = ''
-      assertPathNotExists home-files/.config/bottom
+    nmt.script = let
+      configDir = if pkgs.stdenv.isDarwin then
+        "home-files/Library/Application Support"
+      else
+        "home-files/.config";
+    in ''
+      assertPathNotExists ${configDir}/bottom
     '';
   };
 }

tests/modules/programs/bottom/example-settings-expected.toml

@@ -0,0 +1,6 @@
+[colors]
+low_battery_color = "red"
+
+[flags]
+avg_cpu = true
+temperature_type = "c"

tests/modules/programs/bottom/example-settings.nix

@@ -26,16 +26,7 @@ with lib;
     in ''
       assertFileContent \
         "${configDir}/bottom/bottom.toml" \
-        ${
-          builtins.toFile "example-settings-expected.toml" ''
-            [colors]
-            low_battery_color = "red"
-
-            [flags]
-            avg_cpu = true
-            temperature_type = "c"
-          ''
-        }
+        ${./example-settings-expected.toml}
     '';
   };
 }