The value of `RAD_SOCKET` influences the outcome of `cargo test`.
Users that set this environment variable to, e.g. point `rad` at
a socket at a custom location, were surprised that execution of
tests would interact with their node.
Split `Home::socket` into two functions. `Home::socket_default` is
"pure" and only dependent on the path represented by `Home`. On the
other hand, `Home::socket_from_env` interprets the environment.
Most users will want to use `Home::socket_from_env`.
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