Improve logging for state machine testing

[thomaseizinger]

Our state machine tests are becoming quite complex and often, it is getting difficult to diagnose the real problem of a failure because proptest isn't able to shrink it all the way. This is usually a bug where some precondition isn't correctly set.

The difficulty is fining out, which one is missing because proptest doesn't say, what it is currently doing (shrink from A -> B for example) or remove a certain transition.

Would you accept a PR that adds log or tracing and adds the corresponding statements?