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/p2p_segwit.py

@@ -14,7 +14,6 @@ from test_framework.blocktools import (
     create_block,
     create_coinbase,
 )
-from test_framework.key import ECKey
 from test_framework.messages import (
     MAX_BIP125_RBF_SEQUENCE,
     CBlockHeader,
@@ -89,6 +88,7 @@ from test_framework.util import (
     assert_raises_rpc_error,
 )
 from test_framework.wallet import MiniWallet
+from test_framework.wallet_util import generate_keypair
 
 
 MAX_SIGOP_COST = 80000
@@ -1448,9 +1448,7 @@ class SegWitTest(BitcoinTestFramework):
 
         # Segwit transactions using uncompressed pubkeys are not accepted
         # under default policy, but should still pass consensus.
-        key = ECKey()
-        key.generate(False)
-        pubkey = key.get_pubkey().get_bytes()
+        key, pubkey = generate_keypair(compressed=False)
         assert_equal(len(pubkey), 65)  # This should be an uncompressed pubkey
 
         utxo = self.utxo.pop(0)
@@ -1544,11 +1542,7 @@ class SegWitTest(BitcoinTestFramework):
 
     @subtest
     def test_signature_version_1(self):
-
-        key = ECKey()
-        key.generate()
-        pubkey = key.get_pubkey().get_bytes()
-
+        key, pubkey = generate_keypair()
         witness_script = key_to_p2pk_script(pubkey)
         script_pubkey = script_to_p2wsh_script(witness_script)