Add a test system for Mahogany

This could allow developers to test Mahogany changes in a VM without
touching their current system.

This minimal system only includes mahogany and the necessary batteries
for Guix to run in a VM. The heavy lifting is done by %simple-os
from (gnu tests).

.guix/modules/test-system.scm

@@ -0,0 +1,31 @@
+(define-module (test-system)
+  #:use-module (guix gexp)
+  #:use-module (gnu)
+  #:use-module (gnu packages base)
+  #:use-module (gnu packages linux)
+  #:use-module (gnu system)
+  #:use-module (gnu tests)
+  #:use-module (gnu services desktop)
+  #:use-module (mahogany-package))
+
+(operating-system
+  (inherit %simple-os)
+  (users (cons (user-account
+                  (name "alice")
+                  (comment "Bob's sister")
+                  (group "users")
+                  (supplementary-groups
+                   '("wheel" "audio" "video"
+                     ;; seatd needs the user to be part of seat group
+                     "seat")))
+                 %base-user-accounts))
+  (packages
+   (list coreutils ; Things like ls
+         procps    ; For ps
+         grep      ; For grep
+         mahogany))
+  (services
+   (append
+    (list
+     (service seatd-service-type))
+    %base-services)))