Microsoft.Activities.UnitTesting is a unit testing framework designed to make it easier to unit test activities and workflow services built with Windows Workflow Foundation (WF4).
The Framework contains Test Hosts which wrap WorkflowInvoker, WorkflowApplication and WorkflowServiceHost
Assert helper classes for asserting out arguments and tracking records
Support for Xaml Injection which allows you to mock or stub activities that you don't want to run when testing
Test Framework independent - you can use it with MSTest, NUnit, xUnit or whatever test framework you like.
For more see Microsoft.Activities.UnitTesting Overview