Use Size not PixelSize in ImageTests.

`Image` now bases the size of a bitmap on `IImage.Size`.

tests/Avalonia.Controls.UnitTests/ImageTests.cs

@@ -169,7 +169,7 @@ public void Arrange_Should_Return_Correct_Size_For_UniformToFill_Stretch()
 
         private IBitmap CreateBitmap(int width, int height)
         {
-            return Mock.Of<IBitmap>(x => x.PixelSize == new PixelSize(width, height));
+            return Mock.Of<IBitmap>(x => x.Size == new Size(width, height));
         }
     }
 }