PR description:
> Fixes core#20934 by using the "sync up" method described in core#20538.
>
> After improving robustness with this approach (commits 1-3), it turned out that there were still some fails, but those were unrelated to zmq: Out of 500 runs, 3 times sync_mempool() or sync_blocks() timed out, which can happen because the trickle relay time has no upper bound -- hence in rare cases, it takes longer than 60s. This is fixed by enabling immediate tx relay on node1 (commit 4), which as a nice side-effect also gives us a rough 2x speedup for the test.
>
> For further details, also see the explanations in the commit messages.
Note that the changes in commit 4, the speedup from 45s to 14s runtime using `noban` permissions, were already applied in Bitcoin ABC in D10311.
This is a backport of core#21008 [1/3]
https://github.com/bitcoin/bitcoin/pull/21008/commits/6014d6e1b5a0dda6e20c2721f0bdb7e6a63ece81