Fixes https://github.com/paritytech/polkadot-sdk/issues/4309

If a new block is generated between these 2 lines:

```
  const proof = await apis[nodeName].rpc.mmr.generateProof([1, 9, 20]);

  const root = await apis[nodeName].rpc.mmr.root()
```

we will try to verify a proof for the previous block with the mmr root
at the current block. Which will fail.

So we generate the proof and get the mmr root at block 21 for
consistency.

* test that successfully verify self-generated proof

* verify proof result on all unpaused nodes

* tighten interface: reuse connection

* generate the proof on arbitrary node

* s/stub-name/validator

seems this doesn't pass some regex

* rename script

* add basic mmr-leaves test

* documentation formatting

* check lower bound on mmr leaves

* test beefy rpc: finalized heads match

* test mmr api: stateless proofs

* set lower bound on number of leaves

* change leaves in proof generation

* remove TODOs

* cleanup: consistently ignore zndsl name arg

* refactor: simplify returns

Co-authored-by: Serban Iorga <serban@parity.io>

* lax finalized head test

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* fixup! refactor: simplify returns

* refactor out getApis

Co-authored-by: Serban Iorga <serban@parity.io>

* split out paused validator from group

* refactor: don't ignore node arg

don't randomize selection of proof generating / finalized head
retrieving node, but use the node arg for this.

* only check min block height, not relative

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* verify finalized heads share same canonical chain

picks the node with the highest finalized head, gets its blockchain
headers since genesis, and verifies that all other nodes' finalized
heads are in said blockchain.

* fixup! split out paused validator from group

Co-authored-by: Serban Iorga <serban@parity.io>

---------

Co-authored-by: Serban Iorga <serban@parity.io>
Co-authored-by: Adrian Catangiu <adrian@parity.io>