Adjusting saga id test for 4.1

Since the 4.1 code base assumes that if a saga id is present only one saga
can be targeted:
https://github.com/Particular/NServiceBus/blob/327b8342c55d819f288690608ffb9c24bd321552/src/NServiceBus.Core/Sagas/SagaDispatcherFactory.cs#L186

this means that the test can't check for the other saga to be loaded. I
belive this is fine since I can't really come up with a use case for a
message with a saga id hitting 2 sagas