Add retry loop for Sniffer upstream connection

When connecting to upstream, the Sniffer now retries on connection
failure instead of panicking. This matches the pattern used in
MockDownstream and SnifferSV1.

Closes #151

Author: xyephy

integration-tests/lib/sniffer.rs

@@ -91,11 +91,17 @@ impl<'a> Sniffer<'a> {
                 create_downstream(wait_for_client(listening_address).await)
                     .await
                     .expect("Failed to create downstream");
-            let (upstream_receiver, upstream_sender) = create_upstream(
-                TcpStream::connect(upstream_address)
-                    .await
-                    .expect("Failed to connect to upstream"),
-            )
+            let (upstream_receiver, upstream_sender) = create_upstream(loop {
+                match TcpStream::connect(upstream_address).await {
+                    Ok(stream) => break stream,
+                    Err(_) => {
+                        println!(
+                            "Sniffer {}: unable to connect to upstream {}, retrying",
+                            identifier, upstream_address
+                        );
+                    }
+                }
+            })
             .await
             .expect("Failed to create upstream");
             select! {