Specify nuget feed on README

[AArnott]

The README file tells us the package ID, but not the feed the package comes from. Since it isn't on nuget.org, this is crucial information to consuming the test framework.

[AArnott]

For others looking for it, I found a version from Feb 2023 on this feed: https://dev.azure.com/azure-public/vside/_artifacts/feed/vssdk-archived/NuGet/Microsoft.VisualStudio.Extensibility.Testing.Xunit/overview/0.1.169-beta