Consider alternatives to Moq

[Recent events](https://github.com/moq/moq/issues/1372) related to the [Moq .NET test mock library](https://github.com/moq/moq) have exposed potential privacy concerns. While versions of the library related to those concerns have been delisted from NuGet, to eliminate any exposure to those concerns in the future, alternatives should be considered (e.g. [NSubstitute](https://github.com/nsubstitute/NSubstitute)).

For example, [here](https://github.com/philliphoff/dotnet-sdk/tree/philliphoff-nsubstitute) is a branch with the required to move to NSubstitute.