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/wallet_send.py

@@ -9,7 +9,6 @@ from itertools import product
 
 from test_framework.authproxy import JSONRPCException
 from test_framework.descriptors import descsum_create
-from test_framework.key import ECKey
 from test_framework.messages import (
     ser_compact_size,
     WITNESS_SCALE_FACTOR,
@@ -22,7 +21,8 @@ from test_framework.util import (
     assert_raises_rpc_error,
     count_bytes,
 )
-from test_framework.wallet_util import bytes_to_wif
+from test_framework.wallet_util import generate_keypair
+
 
 class WalletSendTest(BitcoinTestFramework):
     def add_options(self, parser):
@@ -500,9 +500,7 @@ class WalletSendTest(BitcoinTestFramework):
         assert res["complete"]
 
         self.log.info("External outputs")
-        eckey = ECKey()
-        eckey.generate()
-        privkey = bytes_to_wif(eckey.get_bytes())
+        privkey, _ = generate_keypair(wif=True)
 
         self.nodes[1].createwallet("extsend")
         ext_wallet = self.nodes[1].get_wallet_rpc("extsend")