Add an NU5 block test vector that contains many shielded Sapling transactions

[dconnolly]

Motivation

When blocks started showing up on mainnet after NU5 with many shielded Sapling transactions, we observed aberrant zebrad verification behavior. For similar blocks with many Orchard transactions, we identified some bugs already. We should include these blocks (at least one) as test vectors.

Related Work

https://github.com/ZcashFoundation/zebra/issues/4774

[teor2345]

Zebra's performance is good enough for now, so this is a low priority.