Fix mmr zombienet test (#5417)

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.