Refactor events checks in tests: use `System::assert_has_event`

We should not check all events neither rely on events order. So, this issue is just about replacing all events checks by System::assert_has_event.

For instance, instead of:

        let events = System::events();
        assert_eq!(events.len(), 2);
        assert_eq!(
            events[0],
            EventRecord {
                phase: Phase::Initialization,
                event: Event::Identity(pallet_identity::Event::IdtyCreated {
                    idty_index: 6,
                    owner_key: 6,
                }),
                topics: vec![],
            }
        );
        assert_eq!(
            events[1],
            EventRecord {
                phase: Phase::Initialization,
                event: Event::Cert(pallet_certification::Event::NewCert {
                    issuer: 1,
                    issuer_issued_count: 5,
                    receiver: 6,
                    receiver_received_count: 1
                }),
                topics: vec![],
            }
        );

We should just write:

System::assert_has_event(Event::Identity(pallet_identity::Event::IdtyCreated {
        idty_index: 6,
        owner_key: 6,
}));
System::assert_has_event(Event::Cert(pallet_certification::Event::NewCert {
        issuer: 1,
        issuer_issued_count: 5,
        receiver: 6,
        receiver_received_count: 1
}));