Move signed package testing utils to new package

[zivkan]

Bug

Fixes: https://github.com/NuGet/Client.Engineering/issues/3051
Fixes: https://github.com/NuGet/Client.Engineering/issues/543

Description

NuGetGallery uses some of the signed package utilities from this repo's Test.Utility assembly. However, this project is used as a place to put pretty much any shared test utility in the repo, including for test utility classes for NuGet.Client's VS code. This means the project has project references to some VS assemblies, and therefore when Test.Utility is packed into a package, it has dependencies on packages that don't exist (and have no reason to exist).

So, to clean up the "software architecture" a bit, I created a new assembly, moved all the signed package utilities into that new project, and updated everything.

I named the new project Internal.NuGet.Testing.SignedPackages, because our normal process when publishing to nuget.org is to create an API key for the nuget.* namespace, and by starting this package with internal.*, it reduces the risk that this package accidentally gets pushed to nuget.org.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests n/a
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. n/a

[nkolev92]

There's a bunch of test failures, not sure where that got broken.

The change looks as expected. I'd love to chat about the package name, but we can do that later in the PR meeting.

[agr]

Looks good!

[nkolev92]

This PR fixes https://github.com/NuGet/Client.Engineering/issues/543 as well.