Enable async backing on asset-hub-rococo (#2826)

The goal is to move all system chains on Rococo (+ other testnets?) to
use async backing.

Starting with `asset-hub-rococo` to get feedback, before I do the rest.

## Related

Example: https://github.com/paritytech/polkadot-sdk/pull/1619/
Guide:
https://github.com/w3f/polkadot-wiki/blob/master/docs/maintain/maintain-guides-async-backing.md

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: Dónal Murray <donal.murray@parity.io>

cumulus/parachains/runtimes/bridge-hubs/test-utils/src/test_cases/mod.rs

@@ -45,7 +45,7 @@ use frame_system::pallet_prelude::BlockNumberFor;
 use parachains_common::AccountId;
 use parachains_runtimes_test_utils::{
 	mock_open_hrmp_channel, AccountIdOf, BalanceOf, CollatorSessionKeys, ExtBuilder, RuntimeCallOf,
-	XcmReceivedFrom,
+	SlotDurations, XcmReceivedFrom,
 };
 use sp_runtime::{traits::Zero, AccountId32};
 use xcm::{latest::prelude::*, AlwaysLatest};
@@ -419,6 +419,7 @@ pub fn message_dispatch_routing_works<
 	NetworkDistanceAsParentCount,
 >(
 	collator_session_key: CollatorSessionKeys<Runtime>,
+	slot_durations: SlotDurations,
 	runtime_para_id: u32,
 	sibling_parachain_id: u32,
 	unwrap_cumulus_pallet_parachain_system_event: Box<
@@ -529,6 +530,7 @@ pub fn message_dispatch_routing_works<
 			sibling_parachain_id.into(),
 			included_head,
 			&alice,
+			&slot_durations,
 		);
 		let result =
 			<<Runtime as BridgeMessagesConfig<MessagesPalletInstance>>::MessageDispatch>::dispatch(