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/feature_assumevalid.py

@@ -35,7 +35,6 @@ from test_framework.blocktools import (
     create_block,
     create_coinbase,
 )
-from test_framework.key import ECKey
 from test_framework.messages import (
     CBlockHeader,
     COutPoint,
@@ -46,9 +45,13 @@ from test_framework.messages import (
     msg_headers,
 )
 from test_framework.p2p import P2PInterface
-from test_framework.script import (CScript, OP_TRUE)
+from test_framework.script import (
+    CScript,
+    OP_TRUE,
+)
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal
+from test_framework.wallet_util import generate_keypair
 
 
 class BaseNode(P2PInterface):
@@ -90,9 +93,7 @@ class AssumeValidTest(BitcoinTestFramework):
         self.blocks = []
 
         # Get a pubkey for the coinbase TXO
-        coinbase_key = ECKey()
-        coinbase_key.generate()
-        coinbase_pubkey = coinbase_key.get_pubkey().get_bytes()
+        _, coinbase_pubkey = generate_keypair()
 
         # Create the first block with a coinbase output to our key
         height = 1