Add tests for exotic 12-bit sample images

These images should correctly report their color depths, but fail to
decode as they are unsupported.

tests/decode_images.rs

@@ -44,6 +44,18 @@ fn test_rgb_u8() {
     }
 }
 
+#[test]
+fn test_gray_u12() {
+    let img_file =
+        File::open("./tests/images/12bit.cropped.tiff").expect("Cannot find test image!");
+    let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
+    assert_eq!(decoder.colortype().unwrap(), ColorType::Gray(12));
+    assert!(match decoder.read_image() {
+        Err(tiff::TiffError::UnsupportedError(tiff::TiffUnsupportedError::UnsupportedColorType(_))) => true,
+        _ => false,
+    });
+}
+
 #[test]
 fn test_gray_u16() {
     let img_file =
@@ -104,6 +116,18 @@ fn test_gray_u64() {
     }
 }
 
+#[test]
+fn test_rgb_u12() {
+    let img_file =
+        File::open("./tests/images/12bit.cropped.rgb.tiff").expect("Cannot find test image!");
+    let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
+    assert_eq!(decoder.colortype().unwrap(), ColorType::RGB(12));
+    assert!(match decoder.read_image() {
+        Err(tiff::TiffError::UnsupportedError(tiff::TiffUnsupportedError::UnsupportedColorType(_))) => true,
+        _ => false,
+    });
+}
+
 #[test]
 fn test_rgb_u16() {
     let img_file = File::open("./tests/images/rgb-3c-16b.tiff").expect("Cannot find test image!");