connect_isolated only works on mainnet

[teor2345]

Is your feature request related to a problem? Please describe.

zebra_network::isolated::connect_isolated uses the default zebra_network::Config, which is configured for mainnet.

But the Zcash network protocol uses different magic numbers for mainnet and testnet in its message headers, so connect_isolated will fail on testnet:

zebra/zebra-network/src/protocol/external/codec.rs

Line 161 in 69ba558

header_writer.write_all(&Magic::from(self.builder.network).0[..])?;

Describe the solution you'd like

• Add a Network argument to connect_isolated, and substitute it for the default network in peer::Handshake::builder. The easiest way to do this might be to add a with_network method to peer::Handshake::builder.
• Add tests for the connect_isolated handshake on mainnet and testnet.

There's no need to actually send a transaction as part of the tests - a successful handshake or an informational query should be enough.

Additional context

This change isn't required until we need to send isolated user-generated transactions from zebra-client.