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/test_framework/wallet.py

@@ -20,6 +20,7 @@ from test_framework.address import (
     key_to_p2wpkh,
     output_key_to_p2tr,
 )
+from test_framework.blocktools import COINBASE_MATURITY
 from test_framework.descriptors import descsum_create
 from test_framework.key import (
     ECKey,
@@ -53,7 +54,7 @@ from test_framework.util import (
     assert_equal,
     assert_greater_than_or_equal,
 )
-from test_framework.blocktools import COINBASE_MATURITY
+from test_framework.wallet_util import generate_keypair
 
 DEFAULT_FEE = Decimal("0.0001")
 
@@ -395,9 +396,7 @@ def getnewdestination(address_type='bech32m'):
        'legacy', 'p2sh-segwit', 'bech32' and 'bech32m'. Can be used when a random
        destination is needed, but no compiled wallet is available (e.g. as
        replacement to the getnewaddress/getaddressinfo RPCs)."""
-    key = ECKey()
-    key.generate()
-    pubkey = key.get_pubkey().get_bytes()
+    key, pubkey = generate_keypair()
     if address_type == 'legacy':
         scriptpubkey = key_to_p2pkh_script(pubkey)
         address = key_to_p2pkh(pubkey)