Refactor _commonFuzzerAssumptions method in the FlexVotingClient tests

[apbendi]

The _commonFuzzerAssumptions method should be updated in a few ways for clarity, convention, and test speed:

• Use bound rather than assume for the support type value, cast it to the enum type, and return it from the method (as a tuple along with vote weight)
• Rename _address to _account (nit)
• Rename the method to something more active, maybe _assumeSafeAccountAndBoundVote or similar