more descriptive name for QR_core.bmp + giving AppVeyor a kick

tests/ImageSharp.Tests/TestImages.cs

@@ -116,7 +116,7 @@ namespace ImageSharp.Tests
             public const string Car = "Bmp/Car.bmp";
             public const string F = "Bmp/F.bmp";
             public const string NegHeight = "Bmp/neg_height.bmp";
-            public const string CoreHeader = "Bmp/QR_core.bmp";
+            public const string CoreHeader = "Bmp/BitmapCoreHeaderQR.bmp";
             public static readonly string[] All = { Car, F, NegHeight, CoreHeader };
         }