rust: fix unit test link error on Rust 1.70

rust: fix unit test link error on Rust 1.70

Rust 1.70 appears to now link code on both branches of `if cfg!(test)`
now causing Rust unit tests to fail as that pattern was used to
disable functions only available when linked with the Suricata C code.

To work-around this issue, provide two versions of the `new` function,
one for unit tests and one when running as an application.