[IM6.9][IM6.7] Fix test_image_type() (#479)
[IM6.9][IM6.7] Fix test_image_type() (#479)

We use `GetImageType()` in ImageMagick API to retrieve image type.
The API uses `IsGrayImage()` to detect GrayscaleMatteType.

```
  if (IsGrayImage(image,exception) != MagickFalse)
    {
      if (image->matte != MagickFalse)
        return(GrayscaleMatteType);
      return(GrayscaleType);
    }
```
https://github.com/ImageMagick/ImageMagick6/blob/27b1c74979ac473a430e266ff6c4b645664bc805/magick/attribute.c#L771-L776

We have known that the API have breaking change.
https://github.com/rmagick/rmagick/blob/1c5aed2fd15f2aa5f6ae0227e8c9b7437d11d92d/ext/RMagick/rmimage.c#L7156-L7163

So, I think test_image_type() should not test distinct value.
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick