Add decode test for CMYK(8)

tests/decode_images.rs

@@ -47,6 +47,11 @@ fn test_image_color_type_unsupported(file: &str, expected_type: ColorType) {
     });
 }
 
+#[test]
+fn test_cmyk_u8() {
+    test_image_sum_u8("cmyk-3c-8b.tiff", ColorType::CMYK(8), 8522658);
+}
+
 #[test]
 fn test_gray_u8() {
     test_image_sum_u8("minisblack-1c-8b.tiff", ColorType::Gray(8), 2840893);