fixup! Pass invalid_image_name into each image saving job

These specs pass on Windows, but fail on Travis, so try a different way
of comparing images.

spec/save_images_spec.rb

@@ -76,18 +76,24 @@
       it "creates a dummy invalid image" do
         allow(saving).to receive(:attempt_image_capture).and_raise("boom")
         # Create the image at the original res of the invalid image, so that we
-        # can compare it's md5 easily
+        # can compare its md5 easily
         jobs = [
           ['test', '/test/path', '1500x1500', test_url1, test_image1, nil, nil, nil,
            'invalid1.jpg']
         ]
         saving.parallel_task(jobs)
-        # Test fixture image created from the original invalid image because
-        # the original is a jpg
-        invalid_image = get_path_relative_to __FILE__, "./images/invalid1.png"
-        invalid_md5 = Digest::MD5.hexdigest(File.read(invalid_image))
-        created_md5 = Digest::MD5.hexdigest(File.read(test_image1))
-        expect(created_md5).to eq invalid_md5
+        # Create the expected image so we can compare them, because they'll
+        # differ between imagemagick versions and platforms and so we can't just
+        # use a fixture.
+        saving.create_invalid_image("shots/test/invalid1.png", 1500, "invalid1.jpg")
+        Wraith::CompareImages.new(config_name).compare_task(
+          test_image1,
+          "shots/test/invalid1.png",
+          "shots/test/test_diff.png",
+          "shots/test/test.txt"
+        )
+        diff = File.open("shots/test/test.txt", "rb").read
+        expect(diff).to eq "0.0"
       end
     end
   end