Use lexical-binding in underline.el and add tests

* lisp/textmodes/underline.el: Use lexical-binding.
* test/lisp/textmodes/underline-tests.el: New file.
(Bug#36063)

lisp/textmodes/underline.el

-;;; underline.el --- insert/remove underlining (done by overstriking) in Emacs
+;;; underline.el --- insert/remove underlining (done by overstriking) in Emacs  -*- lexical-binding: t -*-
 
 ;; Copyright (C) 1985, 2001-2019 Free Software Foundation, Inc.
 

test/lisp/textmodes/underline-tests.el

+;;; underline-tests.el --- Tests for underline.el  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2019 Free Software Foundation, Inc.
+
+;; Author: Stefan Kangas <stefankangas@gmail.com>
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs 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.
+
+;; GNU Emacs 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 GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'ert)
+(require 'underline)
+
+(ert-deftest underline-tests-underline-region ()
+  (with-temp-buffer
+    (insert "foo bar baz")
+    (underline-region 5 8)
+    (should (equal (buffer-string) "foo _\C-hb_\C-ha_\C-hr baz"))))
+
+(ert-deftest underline-tests-ununderline-region ()
+  (with-temp-buffer
+    (insert "foo _\C-hb_\C-ha_\C-hr baz")
+    (ununderline-region 5 13)
+    (should (equal (buffer-string) "foo bar baz"))))
+
+(provide 'underline-tests)
+;;; underline-tests.el ends here