According to .NET docs for the InternalsVisibleToAttribute:
The attribute is applied at the assembly level. This means that it can be included at the beginning of a source code file, or it can be included in the AssemblyInfo file in a Visual Studio project.
In other words, you can simply place it in your own arbitrarily named .cs file, and it should work fine:
// some .cs file included in your project
using System.Runtime.CompilerServices;
[assembly:InternalsVisibleTo("MyTests")]