Add test for `self-insert-command' (bug#21633)

* test/automated/cmds-tests.el: New file.

test/automated/cmds-tests.el

@@ -0,0 +1,34 @@
+;;; cmds-tests.el --- Testing some Emacs commands
+
+;; Copyright (C) 2013-2015 Free Software Foundation, Inc.
+
+;; Author: Nicolas Richard <youngfrog@members.fsf.org>
+;; Keywords:
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;
+
+;;; Code:
+
+
+(ert-deftest self-insert-command-with-negative-argument ()
+  "Test `self-insert-command' with a negative argument."
+  (let ((last-command-event ?a))
+    (should-error (self-insert-command -1))))
+
+(provide 'cmds-tests)
+;;; cmds-tests.el ends here