Set explicit version in Test project (#332)

Author: Franklin89

CoreWiki.Test/CoreWiki.Test.csproj

@@ -7,6 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.AspNetCore.App" Version="2.1.5" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.8.0" />
     <PackageReference Include="Moq" Version="4.9.0" />
     <PackageReference Include="xunit" Version="2.4.0" />