adding multiple TargetFrameworks

tests/ImageSharp.Tests/ImageSharp.Tests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.0</TargetFramework>
+    <TargetFrameworks>netcoreapp2.0;net462;net47;net471</TargetFrameworks>
     <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
     <DebugType Condition="$(codecov) != ''">full</DebugType>
     <DebugType Condition="$(codecov) == ''">portable</DebugType>
@@ -16,6 +16,7 @@
     <None Include="PixelFormats\PixelOperationsTests.Blender.cs" />
   </ItemGroup>
   <ItemGroup>
+    <PackageReference Include="Microsoft.CSharp" Version="4.4.1" />
     <PackageReference Include="System.Numerics.Vectors" Version="4.5.0-preview1-26216-02" />
     <PackageReference Include="System.Drawing.Common" Version="4.5.0-preview1-26216-02" />
     <PackageReference Include="xunit" Version="2.3.1" />
@@ -37,6 +38,9 @@
   <ItemGroup>
     <Service Include="{82a7f48d-3b50-4b1e-b82e-3ada8210c358}" />
   </ItemGroup>
+  <ItemGroup>
+    <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.1" />
+  </ItemGroup>
   <ItemGroup>
     <None Update="xunit.runner.json">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>