Remove sc_network::NetworkService::register_notifications_protocol and partially refactor Grandpa tests (#7646)

* Remove sc_network::NetworkService::register_notifications_protocol

* Missing calls to .into()

* Wrong crate name

* [WIP] Fix Grandpa tests

* One more passing

* One more. Two to go.

* This one was actually already passing 🎉



* Last one compiles

* Progress

* grandpa: fix voter_persists_its_votes test

* Restore other tests

* Try spawn future later

Co-authored-by: André Silva <andrerfosilva@gmail.com>