[ecash-herald] Reduce test blocks

Summary:
T2972

See D14099. Adding mocked chronik responses of utxo set makes the mocks file impractically large (~30MB vs less than 1 MB).

Judgment that it is more important to keep these mocks, as they are important to testing functionality of address balance calcs, than to keep a large number of test blocks.

Some other supporting points for removing these test blocks:

1. The sendMsgByBlock script allows testing new features on specific blocks known to have desired txs
2. Impractical to keep expanding test blocks for new features, better to test the features on specific txs; this is the approach taken for the parseOpReturn tests for example
3. We'll never send a msg about the genesis block

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D14114