feat: allow for unit testing of before / after state of simulated transactions

[Yorkemartin]

Devs can use seatbelt as a local testing framework for building proposal transactions. Currently, this requires manually reviewing the local proposal report or the stack trace of a transaction in a json format.

Allowing devs to create unit tests that check expected outputs against the stack trace of a simulated proposal transaction will allow devs more independence in their development process, and give the community a clearer picture of the security of a given proposal leading up to, and during voting period.