Tests for on-demand relays

[svyatonik]

Follow up for #2129 and #2135. Let's start implementing new TestingClient and add missing tests for on-demand relays (parachain and headers).

[svyatonik]

The prototype is in this branch: https://github.com/paritytech/parity-bridges-common/compare/test-client-prototype

The biggest problem is that at the client level we don't know what the transaction is about and can't update our testing state accordingly (this is also slightly related to the #2446). Possible solutions are:

• to be able to decode contents of every encoded transaction, parse it and e.g. update best-finalized-bridged-chain-header. This isn't a perfect solution, because we'll need such code for every chain we want to use in tests;
• to use some mocked call builders, which will generate unified calls for all chains. The test client would need to decode this call and get what he needs. This is also not perfect, because we may want to test actual wire transactions contents.

So I'm still exploring

UPD: concept is here: https://github.com/paritytech/parity-bridges-common/compare/relay-level-runtime-calls. It is the (start of) impl of second option from above. Can't open PR, because new client is currently reverted

[svyatonik]

So the solution for previous comment is definitely introducing some relay-level calls and not only calls - we also want to read storage values with meaningful keys. So at client level instead of fn raw_storage_value(key: Vec<u8>), we'll need to have something like fn raw_storage_value(key: enum { WithTestChainA.GrandpaBridge.IsPalletOperational, WithTestChainB.GrandpaBridge.BestFinalized, ... }). Otherwise e.g. test client implementation will be very complex.

So some intermediate "results" of working on this issue (I'll probably take a break here and switch on some other task):

• branch https://github.com/paritytech/parity-bridges-common/compare/test-client-prototype has prototype test for on-demand headers relay. It doesn't work, because fn raw_storage_value() is not implemented by test client and implementing it properly requires a lot of efforts (see next point);
• branch https://github.com/paritytech/parity-bridges-common/compare/relay-level-runtime-calls is an attempt to introduce meaningful calls instead of call: Vec<u8> at the client level. It needs huge refactoring, though - we need a "convertor" that is able to convert all possible calls from RuntimeCall<BridgedChain> type into runtime-level calls. This shall include batch transactions. So basically all the CallBuilders stuff needs to be changed - that's a lot.

[EmmanuellNorbertTulbure]

@svyatonik to re-check and continue if needed

[svyatonik]

superseded by #3002