refactor(gossipsub): use dummy handler instead of calling `new_handle…

[thomaseizinger]

…r` (libp2p#3384)

The gossipsub tests are calling lifecycle functions of the NetworkBehaviour that aren't meant to be called outside of Swarm. This already surfaced as a problem in libp2p#3327 and it is coming up again in libp2p#3254 where new_handler gets deprecated.

Try to mitigate that by constructing a dummy handler instead. Functionally, there is no difference as in both cases, the given handler has never seen a connection.## Description

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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