fix(wpcarro/emacs): Fix list.el tests

I was getting false-positive ERT test results because I forgot to use the
`should` macro in my assertions. I discovered this when debugging a subtle bug
in cycle.el that depends on `list-contains?` return `t` or `nil` instead of
truthy or falsy values.

Change-Id: Ibbf89fd1c4f50f86d5efcaa4cd87280b97e111ce
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6003
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/emacs/pkgs/list/default.nix

@@ -12,6 +12,7 @@ let
             dash
           ]) ++
           (with depot.users.wpcarro.emacs.pkgs; [
+            maybe
             set
           ]);
       })