Improve FlexVotingClient test structure (#71)

* Add constructor test

* Isolate expressVote tests

* Rename and reorder CastVote tests

* Tests for _rawBalanceOf and _castVoteReasonString

* Add tests to fill out spec

* Spec out getPastRawBalance

* Fix bounds for second amounts

* Rename fuzzing helper function

* Fix block number for checks in getPastRawBalance test

* Use bound instead of assume for uint48s

* Refactor FlexClient assumption helpers, fixes #74

* Clarify reasons for vote bounds throughout tests

* Ensure expressVote reverts for unknown proposals

* More assumption cleanup

* Define a MAX_VOTE_TYPE to simplify code

* Pin foundry to get CI to pass

* forge fmt

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
       - name: Install Foundry
         uses: foundry-rs/foundry-toolchain@v1
         with:
-          version: nightly
+          version: nightly-e649e62f125244a3ef116be25dfdc81a2afbaf2a
 
       - name: Run tests
         run: forge test