Use QCheck2 in the mempool tests

Thanks to @vch9 initial work for supporting QCheck2 in !3657 (merged), it is now a low-hanging fruit to update existing tests to QCheck2. Note that this is not for merely the pleasure of having a shiny new toy. QCheck2 brings automatic shrinking, which means that, when a test fails; QCheck2 will try hard to find the smallest example that triggers the failure (starting from the data that triggered the initial failure, it shrinks it, reproduce the failure, iterate). Doing this task will help augment developer productivity significantly as soon as a mempool PBT test starts failing.

This issue is part of #1583.