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_util.py

@@ -63,12 +63,9 @@ def get_generate_key():
     """Generate a fresh key
 
     Returns a named tuple of privkey, pubkey and all address and scripts."""
-    eckey = ECKey()
-    eckey.generate()
-    privkey = bytes_to_wif(eckey.get_bytes())
-    pubkey = eckey.get_pubkey().get_bytes().hex()
+    privkey, pubkey = generate_keypair(wif=True)
     return Key(privkey=privkey,
-               pubkey=pubkey,
+               pubkey=pubkey.hex(),
                p2pkh_script=key_to_p2pkh_script(pubkey).hex(),
                p2pkh_addr=key_to_p2pkh(pubkey),
                p2wpkh_script=key_to_p2wpkh_script(pubkey).hex(),
@@ -114,8 +111,14 @@ def bytes_to_wif(b, compressed=True):
         b += b'\x01'
     return byte_to_base58(b, 239)
 
-def generate_wif_key():
-    # Makes a WIF privkey for imports
-    k = ECKey()
-    k.generate()
-    return bytes_to_wif(k.get_bytes(), k.is_compressed)
+def generate_keypair(compressed=True, wif=False):
+    """Generate a new random keypair and return the corresponding ECKey /
+    bytes objects. The private key can also be provided as WIF (wallet
+    import format) string instead, which is often useful for wallet RPC
+    interaction."""
+    privkey = ECKey()
+    privkey.generate(compressed)
+    pubkey = privkey.get_pubkey().get_bytes()
+    if wif:
+        privkey = bytes_to_wif(privkey.get_bytes(), compressed)
+    return privkey, pubkey