Updated unit tests due to changes in libpng.

Author: dlemstra

tests/Magick.NET.Tests/Formats/Png/PngReadDefinesTests/TheChunkMallocMaxProperty.cs

@@ -43,7 +43,7 @@ public void ShouldLimitTheChunkSize()
             };
             image.Read(Files.SnakewarePNG, settings);
 
-            Assert.Contains("IHDR: chunk data is too large", warning);
+            Assert.Contains("tEXt: out of memory", warning);
         }
     }
 }