Add test for default image not animated

Author: SpaceCheetah

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -88,7 +88,8 @@ public partial class PngDecoderTests
         TestImages.Png.DisposePreviousFirst,
         TestImages.Png.DisposeBackgroundBeforeRegion,
         TestImages.Png.BlendOverMultiple,
-        TestImages.Png.FrameOffset
+        TestImages.Png.FrameOffset,
+        TestImages.Png.DefaultNotAnimated
     };
 
     [Theory]

tests/ImageSharp.Tests/TestImages.cs

@@ -74,6 +74,7 @@ public static class Png
         public const string DisposePreviousFirst = "Png/animated/12-dispose-prev-first.png";
         public const string BlendOverMultiple = "Png/animated/21-blend-over-multiple.png";
         public const string FrameOffset = "Png/animated/frame-offset.png";
+        public const string DefaultNotAnimated = "Png/animated/default-not-animated.png";
         public const string Issue2666 = "Png/issues/Issue_2666.png";
 
         // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html