Based on this post from John Wiegley:
From: "John Wiegley" <johnw@newartisans.com>
Subject: Re: [Emacs-diffs] master
1f02cbe: Fix bug#21766 and add test
To: Juanma Barranquero <lekktu@gmail.com>
Cc: emacs-diffs@gnu.org, bruce.connor.am@gmail.com,
emacs-devel <emacs-devel@gnu.org>
Date: Wed, 28 Oct 2015 18:45:29 -0700
Message-ID: <m2y4emqwg6.fsf@newartisans.com>
https://lists.gnu.org/archive/html/emacs-devel/2015-10/msg02372.html
** Test your changes.
Please test your changes before committing them or sending them to the
-list.
+list. If possible, add a new test along with any bug fix or new
+functionality you commit (of course, some changes cannot be easily
+tested).
Emacs uses ERT, Emacs Lisp Regression Testing, for testing. See (info
"(ert)") or https://www.gnu.org/software/emacs/manual/html_node/ert/