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_nulldummy.py

@@ -35,8 +35,7 @@ from test_framework.util import (
     assert_raises_rpc_error,
 )
 from test_framework.wallet import getnewdestination
-from test_framework.key import ECKey
-from test_framework.wallet_util import bytes_to_wif
+from test_framework.wallet_util import generate_keypair
 
 NULLDUMMY_ERROR = "non-mandatory-script-verify-flag (Dummy CHECKMULTISIG argument must be zero)"
 
@@ -71,12 +70,9 @@ class NULLDUMMYTest(BitcoinTestFramework):
         return tx_from_hex(signedtx["hex"])
 
     def run_test(self):
-        eckey = ECKey()
-        eckey.generate()
-        self.privkey = bytes_to_wif(eckey.get_bytes())
-        self.pubkey = eckey.get_pubkey().get_bytes().hex()
-        cms = self.nodes[0].createmultisig(1, [self.pubkey])
-        wms = self.nodes[0].createmultisig(1, [self.pubkey], 'p2sh-segwit')
+        self.privkey, self.pubkey = generate_keypair(wif=True)
+        cms = self.nodes[0].createmultisig(1, [self.pubkey.hex()])
+        wms = self.nodes[0].createmultisig(1, [self.pubkey.hex()], 'p2sh-segwit')
         self.ms_address = cms["address"]
         ms_unlock_details = {"scriptPubKey": address_to_scriptpubkey(self.ms_address).hex(),
                              "redeemScript": cms["redeemScript"]}