fix(tests): bind to unique ports in ibc tests

Follow-up to #4747. Noticed the new tests were flaking across multiple
runs. Turns out they were fighting for port binds, as described already
in #4517. Let's just use a unique port pair for the MockRelayers and
call it a day for now.

crates/core/app/tests/common/ibc_tests/node.rs

@@ -96,7 +96,9 @@ impl TestNodeWithIBC {
             "b" => 1,
             _ => unreachable!("update this hack"),
         };
-        let grpc_url = format!("http://127.0.0.1:808{}", index) // see #4517
+        // We use a non-standard port range, to avoid conflicting with other
+        // integration tests that bind to the more typical 8080/8081 ports.
+        let grpc_url = format!("http://127.0.0.1:999{}", index) // see #4517
             .parse::<url::Url>()?
             .tap(|url| tracing::debug!(%url, "parsed grpc url"));