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.

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