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_taproot.py

@@ -97,6 +97,7 @@ from test_framework.util import (
     assert_equal,
     random_bytes,
 )
+from test_framework.wallet_util import generate_keypair
 from test_framework.key import (
     generate_privkey,
     compute_xonly_pubkey,
@@ -1186,11 +1187,8 @@ def spenders_taproot_active():
 
     # Also add a few legacy spends into the mix, so that transactions which combine taproot and pre-taproot spends get tested too.
     for compressed in [False, True]:
-        eckey1 = ECKey()
-        eckey1.set(generate_privkey(), compressed)
-        pubkey1 = eckey1.get_pubkey().get_bytes()
-        eckey2 = ECKey()
-        eckey2.set(generate_privkey(), compressed)
+        eckey1, pubkey1 = generate_keypair(compressed=compressed)
+        eckey2, _ = generate_keypair(compressed=compressed)
         for p2sh in [False, True]:
             for witv0 in [False, True]:
                 for hashtype in VALID_SIGHASHES_ECDSA + [random.randrange(0x04, 0x80), random.randrange(0x84, 0x100)]: