message: Add an extra roundtrip test for gray data
diff: @@ -108,6 +108,22 @@ mod tests {
  assert_eq!(loaded_image, buffer_u8);
  }
  + #[test]
  + fn roundtrip_gray() {
  + #[rustfmt::skip]
  + let buf: [u8; 16] = [
  + 0, 0, 0, 255,
  + 255, 255, 255, 255,
  + 255, 0, 255, 0,
  + 255, 0, 0, 0,
  + ];
  +
  + execute_roundtrip_default(&buf, 4, 4, ColorType::L8);
  + execute_roundtrip_with_subtype(&buf, 4, 4, ColorType::L8, PNMSubtype::ArbitraryMap);
  + execute_roundtrip_with_subtype(&buf, 4, 4, ColorType::L8, PNMSubtype::Graymap(SampleEncoding::Ascii));
  + execute_roundtrip_with_subtype(&buf, 4, 4, ColorType::L8, PNMSubtype::Graymap(SampleEncoding::Binary));
  + }
  +
  #[test]
  fn roundtrip_rgb() {
  #[rustfmt::skip]
  