Fix non-netcore2.0 compatible unit test

tests/ImageSharp.Sandbox46/ImageSharp.Sandbox46.csproj

@@ -18,8 +18,8 @@
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="BitMiracle.LibJpeg.NET" Version="1.4.280" />
-    <PackageReference Include="xunit" Version="2.3.0-beta4-build3742" />
-    <PackageReference Include="Moq" Version="4.7.99" />
+    <PackageReference Include="xunit" Version="2.3.1" />
+    <PackageReference Include="Moq" Version="4.8.1" />
     <!--<PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />-->
     <!--<PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />-->
   </ItemGroup>

tests/ImageSharp.Tests/PixelFormats/PixelOperationsTests.cs

@@ -387,13 +387,13 @@ namespace SixLabors.ImageSharp.Tests.PixelFormats
         internal static void TestOperation<TSource, TDest>(
             TSource[] source,
             TDest[] expected,
-            Action<Span<TSource>, Buffer<TDest>> action)
+            Action<Buffer<TSource>, Buffer<TDest>> action)
             where TSource : struct
             where TDest : struct
         {
-            using (TestBuffers<TSource, TDest> buffers = new TestBuffers<TSource, TDest>(source, expected))
+            using (var buffers = new TestBuffers<TSource, TDest>(source, expected))
             {
-                action(buffers.Source, buffers.ActualDestBuffer);
+                action(buffers.SourceBuffer, buffers.ActualDestBuffer);
                 buffers.Verify();
             }
         }