test: refactor: deduplicate legacy ECDSA signing for tx inputs

There are several instances in functional tests and the framework
(MiniWallet, feature_block.py, p2p_segwit.py) where we create a legacy
ECDSA signature for a certain transaction's input by doing the following
steps:
    1) calculate the `LegacySignatureHash` with the desired sighash type
    2) create the actual digital signature by calling `ECKey.sign_ecdsa`
       on the signature message hash calculated above
    3) put the DER-encoded result as CScript data push into
       tx input's scriptSig

Create a new helper `sign_input_legacy` which hides those details and
takes only the necessary parameters (tx, input index, relevant
scriptPubKey, private key, sighash type [SIGHASH_ALL by default]). For
further convenience, the signature is prepended to already existing
data-pushes in scriptSig, in order to avoid rehashing the transaction
after calling the new signing function.

test/functional/feature_block.py

@@ -43,8 +43,7 @@ from test_framework.script import (
     OP_INVALIDOPCODE,
     OP_RETURN,
     OP_TRUE,
-    SIGHASH_ALL,
-    LegacySignatureHash,
+    sign_input_legacy,
 )
 from test_framework.script_util import (
     script_to_p2sh_script,
@@ -539,12 +538,8 @@ class FullBlockTest(BitcoinTestFramework):
             # second input is corresponding P2SH output from b39
             tx.vin.append(CTxIn(COutPoint(b39.vtx[i].sha256, 0), b''))
             # Note: must pass the redeem_script (not p2sh_script) to the signature hash function
-            (sighash, err) = LegacySignatureHash(redeem_script, tx, 1, SIGHASH_ALL)
-            sig = self.coinbase_key.sign_ecdsa(sighash) + bytes(bytearray([SIGHASH_ALL]))
-            scriptSig = CScript([sig, redeem_script])
-
-            tx.vin[1].scriptSig = scriptSig
-            tx.rehash()
+            tx.vin[1].scriptSig = CScript([redeem_script])
+            sign_input_legacy(tx, 1, redeem_script, self.coinbase_key)
             new_txs.append(tx)
             lastOutpoint = COutPoint(tx.sha256, 0)
 
@@ -1338,8 +1333,7 @@ class FullBlockTest(BitcoinTestFramework):
         if (scriptPubKey[0] == OP_TRUE):  # an anyone-can-spend
             tx.vin[0].scriptSig = CScript()
             return
-        (sighash, err) = LegacySignatureHash(spend_tx.vout[0].scriptPubKey, tx, 0, SIGHASH_ALL)
-        tx.vin[0].scriptSig = CScript([self.coinbase_key.sign_ecdsa(sighash) + bytes(bytearray([SIGHASH_ALL]))])
+        sign_input_legacy(tx, 0, spend_tx.vout[0].scriptPubKey, self.coinbase_key)
 
     def create_and_sign_transaction(self, spend_tx, value, script=CScript([OP_TRUE])):
         tx = self.create_tx(spend_tx, 0, value, script)