fix(tests): gate certificate import behind rcgen feature

The `dimpl::certificate` module requires the `rcgen` feature.
Without the cfg gate, CI jobs using `rust-crypto` or `aws-lc-rs`
features alone fail with E0432.

Author: HMBSbige

tests/dtls12/edge.rs

@@ -3,6 +3,7 @@
 use std::sync::Arc;
 use std::time::{Duration, Instant};
 
+#[cfg(feature = "rcgen")]
 use dimpl::certificate::generate_self_signed_certificate;
 use dimpl::{Dtls, Output};
 

tests/dtls13/edge.rs

@@ -3,6 +3,7 @@
 use std::sync::Arc;
 use std::time::{Duration, Instant};
 
+#[cfg(feature = "rcgen")]
 use dimpl::certificate::generate_self_signed_certificate;
 use dimpl::{Config, Dtls, Output};