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/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
+    '';
+  };
+}