Added Microsoft.NET.Test.Sdk dep so NUnit tests appear in Test Explorer

Author: KevinHoward

src/ServiceStack.OrmLite.SqlServerTests/ServiceStack.OrmLite.SqlServerTests.csproj

@@ -13,6 +13,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.3.0" />
     <PackageReference Include="NUnit" Version="3.7.1" />
     <Reference Include="..\..\lib\pcl\ServiceStack.Interfaces.dll" />
   </ItemGroup>