Add reference to System.Runtime.Extensions project in Debug tests

Needed to avoid problematic reference to the Runtime.Extensions Nuget package pulled in by the xunit beta

Author: mellinoe

src/System.Diagnostics.Debug/tests/System.Diagnostics.Debug.Tests.csproj

@@ -24,6 +24,7 @@
       <Name>System.Diagnostics.Debug</Name>
     </ProjectReference>
     <ProjectReference Include="..\..\System.Runtime\src\System.Runtime.CoreCLR.csproj" />
+    <ProjectReference Include="..\..\System.Runtime.Extensions\src\System.Runtime.Extensions.CoreCLR.csproj" />
   </ItemGroup>
   <ItemGroup>
     <None Include="project.json" />