Parameterize the p2p fixture handshake relay flag.

[echennells]

handshake sends relay=false, and session_peer attaches the transaction protocols only when the peer's version sets relay, so no test can reach protocol_transaction_out_106 or protocol_transaction_in_106.

Adds a relay argument defaulted to the current value, so existing cases are unchanged.

peer_setup_fixture in libbitcoin-network has the same literal.

[echennells]

Closing — subsumed, and this branch got there first.

The relay parameter on p2p_setup_fixture::handshake is included verbatim in the not_found rework (notfound-70001, now the head of libbitcoin#1106): same signature, same relay=false default, same out.relay = relay.

It is needed there because protocol_transaction_out_70001 only attaches when the peer's version carries the relay flag, so without this parameter the transaction half of not_found cannot be exercised by a test at all. libbitcoin#1106 uses it for functional_p2p__get_data__unknown_transaction__not_found.

Nothing is lost by closing; the change ships as part of that PR.