fix(relay): ensure stable connect test

[thomaseizinger]

Description

With the changes from #3767, we made the connect test flaky because the Swarm was fully passed to the future and thus dropped as soon as the connection was established. We pass a mutable reference instead which keeps the Swarm alive.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates