Add Visual Studio test project service include marker

Fix #886

LibGit2Sharp.Tests/LibGit2Sharp.Tests.csproj

@@ -129,6 +129,7 @@
   </PropertyGroup>
   <Import Project="..\LibGit2Sharp\CopyWindowsNativeDependencies.targets" />
   <Import Project="..\LibGit2Sharp\ExtraDefine.targets" />
+  <Import Project="VisualStudio.Tests.targets" />
   <PropertyGroup>
     <PreBuildEvent>
     </PreBuildEvent>

LibGit2Sharp.Tests/VisualStudio.Tests.targets

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup>
+    <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />
+  </ItemGroup>
+</Project>