Proper tests for on-demand relays

[svyatonik]

Currently all relays (finality, parachains, messages) are well isolated from the Substrate client and thus have good test coverage. On-demand relays, otoh, are using Substate Client directly, so no proper tests. Instead, they have some fn, which contains almost all relay logic and is not dependent on the Client. But this almost is not enough.

Since on-demand relays complexity has increased recently (with adding on-demand parachains relay), I suggest to introduce some local trait (which is impl for Client) and to add proper tests for both on-demand relays.

[svyatonik]

superseded by #3002