Fix problem of image-tests.el on emba

* test/README: Mention $EMACS_TEST_DIRECTORY.

* test/lisp/image-tests.el (image-tests--emacs-images-directory):
Use `data-directory', for runs w/o of make.
(image-type/from-filename): Check for `image-load-path'.

test/README

@@ -22,7 +22,10 @@ following tags are recognized:
 * :unstable
   The test is under development.  It shall run on demand only.
 
-The Makefile in this directory supports the following targets:
+The Makefile sets the environment variable $EMACS_TEST_DIRECTORY,
+which points to this directory.  This environment variable does not
+exist when the tests are run outside make.  The Makefile supports the
+following targets:
 
 * make check
   Run all tests as defined in the directory.  Expensive and unstable
@@ -113,6 +116,7 @@ Some optional tests require packages from GNU ELPA.  By default
 out somewhere else, use
 
     make GNU_ELPA_DIRECTORY=/path/to/elpa ...
+
 
 There are also continuous integration tests on
 <https://hydra.nixos.org/jobset/gnu/emacs-trunk> (see

test/lisp/image-tests.el

@@ -25,7 +25,7 @@
   (require 'cl-lib))
 
 (defconst image-tests--emacs-images-directory
-  (expand-file-name "../etc/images" (getenv "EMACS_TEST_DIRECTORY"))
+  (expand-file-name "images" data-directory)
   "Directory containing Emacs images.")
 
 (ert-deftest image--set-property ()
@@ -57,6 +57,8 @@
   (should (eq (image-type-from-file-name "foo.png") 'png)))
 
 (ert-deftest image-type/from-filename ()
+  ;; On emba, `image-load-path' does not exist.
+  (skip-unless (bound-and-true-p image-load-path))
   (should (eq (image-type "foo.jpg") 'jpeg)))
 
 (ert-deftest image-type-from-file-header-test ()