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_block.py

@@ -14,7 +14,6 @@ from test_framework.blocktools import (
     get_legacy_sigopcount_block,
     MAX_BLOCK_SIGOPS,
 )
-from test_framework.key import ECKey
 from test_framework.messages import (
     CBlock,
     COIN,
@@ -55,6 +54,7 @@ from test_framework.util import (
     assert_equal,
     assert_greater_than,
 )
+from test_framework.wallet_util import generate_keypair
 from data import invalid_txs
 
 
@@ -98,9 +98,7 @@ class FullBlockTest(BitcoinTestFramework):
         self.bootstrap_p2p()  # Add one p2p connection to the node
 
         self.block_heights = {}
-        self.coinbase_key = ECKey()
-        self.coinbase_key.generate()
-        self.coinbase_pubkey = self.coinbase_key.get_pubkey().get_bytes()
+        self.coinbase_key, self.coinbase_pubkey = generate_keypair()
         self.tip = None
         self.blocks = {}
         self.genesis_hash = int(self.nodes[0].getbestblockhash(), 16)