Add netcoreapp2.0 target to TestHelpers.Tests so that the test can run in unix systems

Author: fgreinacher

TestHelpers.Tests/TestHelpers.Tests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net40</TargetFrameworks>
+    <TargetFrameworks>net40;netcoreapp2.0</TargetFrameworks>
     <Version>0.0.0.1</Version>
     <Description>The unit tests for our pre-built mocks</Description>
     <Company />
@@ -39,6 +39,9 @@
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" 
+                      Version="15.0.0"
+                      Condition="'$(TargetFramework)' != 'net40'" />
     <PackageReference Include="nunit" Version="3.8.1" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.9.0" />
   </ItemGroup>