I was able to get an answer from the team (below). Hope this is helpful (and sorry for the delay).
While to give a full answer, we'd need to know what languages you cared about. the key thing is that it should be pretty easy.
In terms of mocking, just have one method that makes all graph calls and use dependency injection to substitute an arbitrary mock.
You can save the json responses of some real calls and have your mock return those. It should work fine with your favorite mock / DI framework or just make custom mocks.
You can look at the actual dot netsdk for graph for examples: https://github.com/microsoftgraph/msgraph-sdk-dotnet/tree/dev/tests/Microsoft.Graph.Core.Test/Mocks that uses Moq: https://github.com/moq/moq
