lib: allow unused import for ffi-testing feat

Running `cargo clippy-ci` using Rust 1.75 fails:

```
error: unused import: `tests::ffi::*`
  --> src/lib.rs:28:9
   |
28 | pub use tests::ffi::*;
   |         ^^^^^^^^^^^^^
   |
   = note: `-D unused-imports` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unused_imports)]`
```

This commit adds a `allow(unused_imports)` gated on the same
`ffi-testing` feature that the `pub use` statement is conditional upon.

src/lib.rs

@@ -25,6 +25,7 @@ mod tests;
 // Re-export any exported functions that are required for
 // tests to run in a platform-native environment.
 #[cfg(feature = "ffi-testing")]
+#[cfg_attr(feature = "ffi-testing", allow(unused_imports))]
 pub use tests::ffi::*;
 
 /// Creates and returns a `rustls` configuration that verifies TLS