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/mempool_dust.py

@@ -5,7 +5,6 @@
 """Test dust limit mempool policy (`-dustrelayfee` parameter)"""
 from decimal import Decimal
 
-from test_framework.key import ECKey
 from test_framework.messages import (
     COIN,
     CTxOut,
@@ -32,6 +31,7 @@ from test_framework.util import (
     get_fee,
 )
 from test_framework.wallet import MiniWallet
+from test_framework.wallet_util import generate_keypair
 
 
 DUST_RELAY_TX_FEE = 3000  # default setting [sat/kvB]
@@ -74,11 +74,8 @@ class DustRelayFeeTest(BitcoinTestFramework):
         self.wallet = MiniWallet(self.nodes[0])
 
         # prepare output scripts of each standard type
-        key = ECKey()
-        key.generate(compressed=False)
-        uncompressed_pubkey = key.get_pubkey().get_bytes()
-        key.generate(compressed=True)
-        pubkey = key.get_pubkey().get_bytes()
+        _, uncompressed_pubkey = generate_keypair(compressed=False)
+        _, pubkey = generate_keypair(compressed=True)
 
         output_scripts = (
             (key_to_p2pk_script(uncompressed_pubkey),          "P2PK (uncompressed)"),