Add tests for private NuGet feeds in package tab-completion

Per @zivkan, there's some additional setup we need to do to configure direct use of feeds like we do in the completion scenarios for 'dotnet add package'. His comments below:

>Was this tested with a private Azure DevOps feed?
>
>By default, apps using NuGet.Protocol, `SourceRepository` will not look for credential providers. A static method needs to be called, and then `SourceRepository` will search for credential providers when it makes requests on feeds that need authentication. Unfortunately, I can't remember the name of the API, so I can't quickly search the code to see if it's being called. But if testing showed that private Azure Artifacts feeds don't work, let me know and I'll look it up.

_Originally posted by @zivkan in https://github.com/dotnet/sdk/pull/42349#issuecomment-2292422914_