network: Enable tests of Notifications

[dmitry-markin]

libp2p upgrade to v0.52.3 brought new Stream abstraction that has new() method private. This made Notifications tests not compile without possibility to make them work. So, they were commented out in #4682 (see the commit).

This issue is about trying to upstream the patch to libp2p that makes Stream::new() method public and restoring the functionality of Notifications tests.

cc @paritytech/networking

[dmitry-markin]

Addressed in #1631 using unsafe { } trick.