notification/tests: Check test state and increase code coverage

[lexnv]

Increase the code coverage for the notification substream validation testing, and ensure the notification state is checked.

Part of accept_fails_due_to_closed_substream test:

litep2p/src/protocol/notification/tests/substream_validation.rs

Line 332 in e0b7118

// TODO: check state

[lexnv]

Thanks so much @Imod7 for tackling this 🙏

Closed by: #360