test: refactor: introduce generate_keypair helper with WIF support

In functional tests it is a quite common scenario to generate fresh
elliptic curve keypairs, which is currently a bit cumbersome as it
involves multiple steps, e.g.:

    privkey = ECKey()
    privkey.generate()
    privkey_wif = bytes_to_wif(privkey.get_bytes())
    pubkey = privkey.get_pubkey().get_bytes()

Simplify this by providing a new `generate_keypair` helper function that
returns the private key either as `ECKey` object or as WIF-string
(depending on the boolean `wif` parameter) and the public key as
byte-string; these formats are what we mostly need (currently we don't
use `ECPubKey` objects from generated keypairs anywhere).

With this, most of the affected code blocks following the pattern above
can be replaced by one-liners, e.g.:

    privkey, pubkey = generate_keypair(wif=True)

Note that after this commit, the only direct uses of `ECKey` remain in
situations where we want to set the private key explicitly, e.g. in
MiniWallet (test/functional/test_framework/wallet.py) or the test for
the signet miner script (test/functional/tool_signet_miner.py).

test/functional/mempool_accept.py

@@ -9,7 +9,6 @@ from decimal import Decimal
 import math
 
 from test_framework.test_framework import BitcoinTestFramework
-from test_framework.key import ECKey
 from test_framework.messages import (
     MAX_BIP125_RBF_SEQUENCE,
     COIN,
@@ -44,6 +43,7 @@ from test_framework.util import (
     assert_raises_rpc_error,
 )
 from test_framework.wallet import MiniWallet
+from test_framework.wallet_util import generate_keypair
 
 
 class MempoolAcceptanceTest(BitcoinTestFramework):
@@ -283,9 +283,7 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
             rawtxs=[tx.serialize().hex()],
         )
         tx = tx_from_hex(raw_tx_reference)
-        key = ECKey()
-        key.generate()
-        pubkey = key.get_pubkey().get_bytes()
+        _, pubkey = generate_keypair()
         tx.vout[0].scriptPubKey = keys_to_multisig_script([pubkey] * 3, k=2)  # Some bare multisig script (2-of-3)
         self.check_mempool_result(
             result_expected=[{'txid': tx.rehash(), 'allowed': False, 'reject-reason': 'bare-multisig'}],