Add test to ensure the chunk type values are correct

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

@@ -0,0 +1,29 @@
+using System;
+using System.Buffers.Binary;
+using System.Text;
+using SixLabors.ImageSharp.Formats.Png;
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Formats.Png
+{
+    public class PngChunkTests
+    {
+        [Fact]
+        public void ChunkTypeIdsAreCorrect()
+        {
+            Assert.Equal(PngChunkType.Header,       GetType("IHDR"));
+            Assert.Equal(PngChunkType.Palette,      GetType("PLTE"));
+            Assert.Equal(PngChunkType.Data,         GetType("IDAT"));
+            Assert.Equal(PngChunkType.End,          GetType("IEND"));
+            Assert.Equal(PngChunkType.PaletteAlpha, GetType("tRNS"));
+            Assert.Equal(PngChunkType.Text,         GetType("tEXt"));
+            Assert.Equal(PngChunkType.Gamma,        GetType("gAMA"));
+            Assert.Equal(PngChunkType.Physical,     GetType("pHYs"));
+        }
+
+        private static PngChunkType GetType(string text)
+        {
+            return (PngChunkType)BinaryPrimitives.ReadInt32BigEndian(Encoding.UTF8.GetBytes(text).AsSpan());
+        }
+    }
+}