Move assembly attributes to props file

# Background
To facilitate testing, we define the `[InternalsVisibleTo]` attribute on the assembly of each project.

# Problem
We're currently use C# to do this, which puts another file into the solution explorer for every experiment.

# Solution
We can define `InternalsVisibleTo` via MSBuild props file. We should do this to clean up the solution explorer for the user.

Relevant documentation:
https://learn.microsoft.com/en-us/dotnet/core/project-sdk/msbuild-props#internalsvisibleto