fix: import Duration in requests test module

Author: jlizen

lambda-runtime/src/requests.rs

@@ -186,6 +186,7 @@ impl IntoRequest for EventErrorRequest<'_> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::time::Duration;
 
     #[test]
     fn test_next_event_request() {