Add overloads that take actions when testing sagas.

I've found the lack of overloads that accept actions in testing to be an annoyance in an otherwise great product.  The reason I want to use Action is so that I can use NUnit assertions which provide more precise error information than just "the predicate failed somewhere."  (I often times have to make assertions on 5 or more message properties.)

I've done Sagas first and will be happy to do Handlers similarly if this contrib makes the team happy.  I added tests for my overloads only where tests for the corresponding Func<T, bool>  has them.