artresizer: fix image comparison test

Since the ImageMagick based comparison is now abstracted via ArtResizer,
it becomes a little more involved to mock it for testing.

beets/art.py

@@ -121,7 +121,13 @@ def resize_image(log, imagepath, maxwidth, quality):
     return imagepath
 
 
-def check_art_similarity(log, item, imagepath, compare_threshold):
+def check_art_similarity(
+    log,
+    item,
+    imagepath,
+    compare_threshold,
+    artresizer=None,
+):
     """A boolean indicating if an image is similar to embedded item art.
 
     If no embedded art exists, always return `True`. If the comparison fails
@@ -135,7 +141,10 @@ def check_art_similarity(log, item, imagepath, compare_threshold):
         if not art:
             return True
 
-        return ArtResizer.shared.compare(art, imagepath, compare_threshold)
+        if artresizer is None:
+            artresizer = ArtResizer.shared
+
+        return artresizer.compare(art, imagepath, compare_threshold)
 
 
 def extract(log, outpath, item):

test/test_embedart.py

@@ -24,7 +24,7 @@ from test.helper import TestHelper
 
 from mediafile import MediaFile
 from beets import config, logging, ui
-from beets.util import syspath, displayable_path
+from beets.util import artresizer, syspath, displayable_path
 from beets.util.artresizer import ArtResizer
 from beets import art
 
@@ -216,16 +216,31 @@ class EmbedartCliTest(_common.TestCase, TestHelper):
         self.assertEqual(mediafile.images[0].data, self.image_data)
 
 
+class DummyArtResizer(ArtResizer):
+    """An `ArtResizer` which pretends that ImageMagick is available, and has
+    a sufficiently recent version to support image comparison.
+    """
+    @staticmethod
+    def _check_method():
+        return artresizer.IMAGEMAGICK, (7, 0, 0), True
+
+
 @patch('beets.util.artresizer.subprocess')
 @patch('beets.art.extract')
 class ArtSimilarityTest(unittest.TestCase):
     def setUp(self):
         self.item = _common.item()
         self.log = logging.getLogger('beets.embedart')
+        self.artresizer = DummyArtResizer()
 
     def _similarity(self, threshold):
-        return art.check_art_similarity(self.log, self.item, b'path',
-                                        threshold)
+        return art.check_art_similarity(
+            self.log,
+            self.item,
+            b'path',
+            threshold,
+            artresizer=self.artresizer,
+        )
 
     def _popen(self, status=0, stdout="", stderr=""):
         """Create a mock `Popen` object."""