bottom: add module

Bottom is a cross-platform graphical process/system monitor with a
customizable interface and a multitude of features.

Two unit tests were added validate the module behavior for an empty
configuration and the example configuration.

tests/modules/programs/bottom/default.nix

@@ -0,0 +1,4 @@
+{
+  bottom-empty-settings = ./empty-settings.nix;
+  bottom-example-settings = ./example-settings.nix;
+}

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

@@ -0,0 +1,16 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.bottom = {
+      enable = true;
+      package = pkgs.writeScriptBin "dummy" "";
+    };
+
+    nmt.script = ''
+      assertPathNotExists home-files/.config/bottom
+    '';
+  };
+}

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

@@ -0,0 +1,41 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.bottom = {
+      enable = true;
+      package = pkgs.writeShellScriptBin "dummy" "";
+
+      settings = {
+        flags = {
+          avg_cpu = true;
+          temperature_type = "c";
+        };
+
+        colors = { low_battery_color = "red"; };
+      };
+    };
+
+    nmt.script = let
+      configDir = if pkgs.stdenv.isDarwin then
+        "home-files/Library/Application Support"
+      else
+        "home-files/.config";
+    in ''
+      assertFileContent \
+        "${configDir}/bottom/bottom.toml" \
+        ${
+          builtins.toFile "example-settings-expected.toml" ''
+            [colors]
+            low_battery_color = "red"
+
+            [flags]
+            avg_cpu = true
+            temperature_type = "c"
+          ''
+        }
+    '';
+  };
+}