Expose internals in csproj with InternalsVisibleTo

  <ItemGroup>
    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleToAttribute">
    <_Parameter1>ClassLibrary1.Tests</_Parameter1>
    </AssemblyAttribute>
  </ItemGroup>

Update 2024

<ItemGroup>
    <InternalsVisibleTo Include="ClassLibrary1.Tests" />
</ItemGroup>