Merge bitcoin/bitcoin#33446: rpc: fix getblock(header) returns target for tip

bf7996cbc3 rpc: fix getblock(header) returns target for tip (Sjors Provoost)
4c3c1f42cf test: add block 2016 to mock mainnet (Sjors Provoost)

Pull request description:

  A `target` field was added to the `getblock` and `getblockheader` RPC calls in #31583, but it mistakingly always used the tip value.

  This PR fixes it to return the target for the given block. Because regtest does not have difficulty adjustment, the mainnet test is expanded to cover the fix.

  A preliminary commit deals with mining block 2016 that's needed for the test. It also:
  - renames the `create_coinbase` `retarget_period` argument to `halving_period`. Before #31583 this was hardcoded for regtest where these values are the same.
  - drops unused `fees` argument from `mine` helper
  - expands the CPU miner instructions for generating the alternative mainnet chain

  Fixes #33440

ACKs for top commit:
  sipa:
    utACK bf7996cbc3
  luke-jr:
    crACK bf7996cbc3
  TheCharlatan:
    ACK bf7996cbc3
  ismaelsadeeq:
    Code review ACK bf7996cbc3

Tree-SHA512: 2a2e11efd91f4aaccf9d2ec4dff9fd82c366b8a7e797ce5981dca2e6f08028f69154f4e6a27aef20d78b0e6c3304416789267c2fad42d7aa5072f8537d0c8b0d

test/functional/test_framework/blocktools.py

@@ -144,7 +144,7 @@ def script_BIP34_coinbase_height(height):
     return CScript([CScriptNum(height)])
 
 
-def create_coinbase(height, pubkey=None, *, script_pubkey=None, extra_output_script=None, fees=0, nValue=50, retarget_period=REGTEST_RETARGET_PERIOD):
+def create_coinbase(height, pubkey=None, *, script_pubkey=None, extra_output_script=None, fees=0, nValue=50, halving_period=REGTEST_RETARGET_PERIOD):
     """Create a coinbase transaction.
 
     If pubkey is passed in, the coinbase output will be a P2PK output;
@@ -158,7 +158,7 @@ def create_coinbase(height, pubkey=None, *, script_pubkey=None, extra_output_scr
     coinbaseoutput = CTxOut()
     coinbaseoutput.nValue = nValue * COIN
     if nValue == 50:
-        halvings = int(height / retarget_period)
+        halvings = int(height / halving_period)
         coinbaseoutput.nValue >>= halvings
         coinbaseoutput.nValue += fees
     if pubkey is not None: