Partially remove exiftool dependency from image-dired.el

* lisp/image-dired.el (exif): Require.
(image-dired-cmd-read-exif-data-program)
(image-dired-cmd-read-exif-data-options)
(image-dired-get-exif-data): Make obsolete in favour of using
exif.el.  This removes a dependency on external exiftool for some
operations.
(image-dired-get-exif-file-name)
(image-dired-thumbnail-set-image-description): Use exif.el
functions instead of exiftool.
* lisp/image/exif.el (exif-tag-alist): Add description and
copyright fields.
* test/lisp/image-dired-tests.el: New file.

test/lisp/image-dired-tests.el

@@ -0,0 +1,36 @@
+;;; image-dired-tests.el --- Tests for image-dired.el  -*- lexical-binding: t -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; 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/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'image-dired)
+
+(defun image-dired-test-image-file (name)
+  (expand-file-name
+   name (expand-file-name "data/image"
+                          (or (getenv "EMACS_TEST_DIRECTORY")
+                              "../"))))
+
+(ert-deftest image-dired-tests-get-exif-file-name ()
+  (let ((img (image-dired-test-image-file "black.jpg")))
+    (should (equal (image-dired-get-exif-file-name img)
+                   "2019_09_21_16_22_13_black.jpg"))))
+
+;;; image-dired-tests.el ends here