test: integration testing for proposal checks

[moodysalem]

running it locally is good, but is manual testing, and checks can break

would be good to have some automated tests that verify the results of checks on individual proposals

e.g.: for the verify etherscan check, pass in some proposal data that calls a target with unverified code, and separately pass in a proposal that calls verified code, and check the resulting output of the check

these tests are integration testing because they rely on external apis

[moodysalem]

actually, maybe we should just run the whole thing on pull requests in a mode that does not write to the github repo (which is pretty much integration testing)