Add commission rate transition to PoS VP state machine test

[brentstone]

From a thread of comments on #695:

We can however add a Transition for ValidPosAction::ChangeCommissionRate, for which the model would need to have a pre-condition that ensures that the new rate is less than or equal to the validator's max_commission_rate_change with respect to its last epoch's rate. I think this can be done in in the abstract SM by e.g. looking through self.all_valid_actions().into_iter().rev() for the last rate value (either from a previous ValidPosAction::ChangeCommissionRate or ValidPosAction::InitValidator if it's not been changed before) before the last epoch switch. However, the epoch switch is currently not preserved in the abstract state machine - we only have the current epoch, so we might need to change that first.

Originally posted by @tzemanovic in #695 (comment)

[cwgoes]

@brentstone Is this still applicable?

[brentstone]

@brentstone Is this still applicable?

Yes, this ultimately wasn't covered in #695 and should still be done.

[cwgoes]

Does not apply anymore.