Use OP_0 for BIP34 padding in signet and tests

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.

Author: Sjors

test/functional/test_framework/blocktools.py

@@ -35,7 +35,7 @@
     CScript,
     CScriptNum,
     CScriptOp,
-    OP_1,
+    OP_0,
     OP_RETURN,
     OP_TRUE,
 )
@@ -128,8 +128,8 @@ def add_witness_commitment(block, nonce=0):
 def script_BIP34_coinbase_height(height):
     if height <= 16:
         res = CScriptOp.encode_op_n(height)
-        # Append dummy to increase scriptSig size above 2 (see bad-cb-length consensus rule)
-        return CScript([res, OP_1])
+        # Append dummy to increase scriptSig size to 2 (see bad-cb-length consensus rule)
+        return CScript([res, OP_0])
     return CScript([CScriptNum(height)])