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/test_framework/script.py

@@ -689,6 +689,16 @@ def LegacySignatureHash(*args, **kwargs):
     else:
         return (hash256(msg), err)
 
+def sign_input_legacy(tx, input_index, input_scriptpubkey, privkey, sighash_type=SIGHASH_ALL):
+    """Add legacy ECDSA signature for a given transaction input. Note that the signature
+       is prepended to the scriptSig field, i.e. additional data pushes necessary for more
+       complex spends than P2PK (e.g. pubkey for P2PKH) can be already set before."""
+    (sighash, err) = LegacySignatureHash(input_scriptpubkey, tx, input_index, sighash_type)
+    assert err is None
+    der_sig = privkey.sign_ecdsa(sighash)
+    tx.vin[input_index].scriptSig = bytes(CScript([der_sig + bytes([sighash_type])])) + tx.vin[input_index].scriptSig
+    tx.rehash()
+
 # TODO: Allow cached hashPrevouts/hashSequence/hashOutputs to be provided.
 # Performance optimization probably not necessary for python tests, however.
 # Note that this corresponds to sigversion == 1 in EvalScript, which is used