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/p2p_segwit.py

@@ -71,8 +71,8 @@ from test_framework.script import (
     SIGHASH_NONE,
     SIGHASH_SINGLE,
     SegwitV0SignatureHash,
-    LegacySignatureHash,
     hash160,
+    sign_input_legacy,
 )
 from test_framework.script_util import (
     key_to_p2pk_script,
@@ -1529,10 +1529,8 @@ class SegWitTest(BitcoinTestFramework):
         tx5 = CTransaction()
         tx5.vin.append(CTxIn(COutPoint(tx4.sha256, 0), b""))
         tx5.vout.append(CTxOut(tx4.vout[0].nValue - 1000, CScript([OP_TRUE])))
-        (sig_hash, err) = LegacySignatureHash(script_pubkey, tx5, 0, SIGHASH_ALL)
-        signature = key.sign_ecdsa(sig_hash) + b'\x01'  # 0x1 is SIGHASH_ALL
-        tx5.vin[0].scriptSig = CScript([signature, pubkey])
-        tx5.rehash()
+        tx5.vin[0].scriptSig = CScript([pubkey])
+        sign_input_legacy(tx5, 0, script_pubkey, key)
         # Should pass policy and consensus.
         test_transaction_acceptance(self.nodes[0], self.test_node, tx5, True, True)
         block = self.build_next_block()