For blocks 1 through 15 the script_BIP34_coinbase_height appends OP_1
to comply with BIP34 and avoid bad-cb-length.
This is inconsistent with BlockAssembler::CreateNewBlock() which adds
OP_0 instead.
The utxo_total_supply fuzzer and MinerTestingSetup::Block also use OP_0.
Changing it is required to import the test vectors in the next commit.
It also ensures the test vectors can be regenerated using the CPU miner
at https://github.com/pooler/cpuminer without patches (it uses OP_0).
The same helper is used by the signet miner, so this will impact newly
bootstrapped signets.
c4f68c12e2