Add new test file.

The octree quantizer does not like this file. A couple of frame
backgrounds turn out the wrong color. When you resize though all is
fixed!?

tests/ImageSharp.Tests/FileTestBase.cs

@@ -42,6 +42,7 @@ namespace ImageSharp.Tests
             // TestImages.Png.Filter4, // Perf: Enable for local testing only
             // TestImages.Png.FilterVar, // Perf: Enable for local testing only
             TestImages.Gif.Rings,
+            TestImages.Gif.Cheers,
             // TestImages.Gif.Giphy // Perf: Enable for local testing only
         };
 

tests/ImageSharp.Tests/TestImages.cs

@@ -101,6 +101,8 @@ namespace ImageSharp.Tests
 
             public static TestFile Rings => new TestFile(folder + "rings.gif");
             public static TestFile Giphy => new TestFile(folder + "giphy.gif");
+
+            public static TestFile Cheers => new TestFile(folder + "cheers.gif");
         }
     }
 }