Improve bridge hub tests suite in the pokadot-sdk

[svyatonik]

paritytech/polkadot-sdk#2692

What may be improved:

• there's too much generic constraints - think we may use some helper traits for that and then just simply do Runtime: BridgeHubRuntimeWithRemoteGrandpa<GI, MI>, ...;
• there are still some constants, which may cause test failures (e.g. somewhere balance is endowed by 1000 * ED) - let's try to remove them.