tests: allow-list a controller heartbeat error (#8471)

## Problem

`test_change_pageserver` stops pageservers in a way that can overlap
with the controller's heartbeats: the controller can get a heartbeat
success and then immediately find the node unavailable. This particular
situation triggers a log that isn't in our current allow-list of
messages for nodes offline

Example:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8339/10048487700/index.html#testresult/19678f27810231df/retries

## Summary of changes

- Add the message to the allow list

storage_controller/src/service.rs

@@ -828,6 +828,8 @@ impl Service {
                                 );
                             }
                             Err(err) => {
+                                // Transition to active involves reconciling: if a node responds to a heartbeat then
+                                // becomes unavailable again, we may get an error here.
                                 tracing::error!(
                                     "Failed to update node {} after heartbeat round: {}",
                                     node_id,