Improve test coverage of the `Notifications` protocol (#13033)

* Add handler and upgrade tests

* Add tests for `behaviour.rs`

* Apply review comments

* Update dependencies

* Apply suggestions from code review

Co-authored-by: Dmitry Markin <dmitry@markin.tech>

* Apply review comments

* Fix clippy

* Update mockall

* Apply review comment

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Co-authored-by: Dmitry Markin <dmitry@markin.tech>