fix(wpcarro/emacs): Remove lingering dash.el dep

CI couldn't catch this because the dependency is discovered at runtime, and
there were no tests consuming `list-concat`. Added the tests and removed the
usage of `-concat`.

Change-Id: I5bcce9aa8170001a9d35c104da09374ab30ffbc5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6043
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/emacs/pkgs/list/list.el

@@ -44,7 +44,7 @@
 
 (defun list-concat (&rest lists)
   "Joins `LISTS' into on list."
-  (apply #'-concat lists))
+  (apply #'append lists))
 
 (defun list-duplicate (n x)
   "Duplicates the given element, X, N times in a list."