gnu: elogind: Use itself as the cgroup controller.

Fixes a regression introduced in
fa67d56541 whereby elogind would fail to
start with:

  Cannot determine cgroup we are running in: No data available
  Out of memory.

thereby preventing log-in altogether.

* gnu/packages/freedesktop.scm (elogind)[arguments]: Pass
"--with-cgroup-controller=elogind".
* gnu/tests/desktop.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.

gnu/packages/freedesktop.scm

@@ -227,6 +227,12 @@ the freedesktop.org XDG Base Directory specification.")
        (list (string-append "--with-udevrulesdir="
                             (assoc-ref %outputs "out")
                             "/lib/udev/rules.d")
+
+             ;; Let elogind be its own cgroup controller, rather than relying
+             ;; on systemd or OpenRC.  By default, 'configure' makes an
+             ;; incorrect guess.
+             "--with-cgroup-controller=elogind"
+
              (string-append "--with-rootprefix="
                             (assoc-ref %outputs "out"))
              (string-append "--with-rootlibexecdir="