Adaptive Inflation: add tests for the vote once we've modified the threshold (Follow-up from "Adaptive inflation: add new per block toggle vote")

The following discussion from !8661 (merged) should be addressed:

• @lrand started a discussion: (+6 comments)

  I'd like to see some tests similar to those with Liquidity Baking, to check that we indeed manage to surpass a threshold within a certain amount of blocks (this implies adding said threshold, as @mbouaziz already commented above)