Fix testing build (#163)

Apparently if you include a dependency to xunit then magically your package must set IsPackable.

Author: einarmo

Cognite.Testing/Cognite.Testing.csproj

@@ -16,14 +16,16 @@
     <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
+    <IsPackable>true</IsPackable>
   </PropertyGroup>
 
   <ItemGroup>
     <None Include="..\LICENSE" Pack="true" Visible="false" PackagePath="" />
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="xunit" Version="2.4.1" />
+    <PackageReference Include="xunit.assert" Version="2.4.1" />
+    <PackageReference Include="xunit.core" Version="2.4.1" />
   </ItemGroup>
 
   <ItemGroup>