* pallet-mmr: extend batch proof verification test
covers all possible 2-leaf combinations now, including current
verification failures that batch proof item count limit is too low
sometimes.
* raise upper bound on proof item number
as described in
https://github.com/paritytech/substrate/issues/11753#issuecomment-1179838174
* test for powerset of leaves
* refactor batch proof verification test
* test all batch proofs for mmr sizes up to n=13
* limit mmr size to reduce batch proof test duration
* use saturating integer addition for proof check
* extract common chain building in batch proof tests
note: right now, since not killing old chain, it keeps growing by 7
blocks for every leaf selection (added after proof generation), hence
heavier to compute.
* only add blocks after a proof generation once
* increase batch proof testing range
* register offchain extensions only once
* fmt & remove unused util