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/test-utils/src/lib.rs

@@ -29,7 +29,6 @@ use frame_support::{
 	weights::Weight,
 };
 use frame_system::pallet_prelude::{BlockNumberFor, HeaderFor};
-use parachains_common::SLOT_DURATION;
 use polkadot_parachain_primitives::primitives::{
 	HeadData, HrmpChannelId, RelayChainBlockNumber, XcmpMessageFormat,
 };
@@ -115,6 +114,11 @@ impl<Runtime: frame_system::Config + pallet_balances::Config + pallet_session::C
 	}
 }
 
+pub struct SlotDurations {
+	pub relay: SlotDuration,
+	pub para: SlotDuration,
+}
+
 /// A set of traits for a minimal parachain runtime, that may be used in conjunction with the
 /// `ExtBuilder` and the `RuntimeHelper`.
 pub trait BasicParachainRuntime:
@@ -335,6 +339,7 @@ impl<
 		open_hrmp_channel: Option<(u32, u32)>,
 		included_head: HeaderFor<Runtime>,
 		slot_digest: &[u8],
+		slot_durations: &SlotDurations,
 	) -> DispatchResult
 	where
 		HrmpChannelOpener: frame_support::inherent::ProvideInherent<
@@ -348,6 +353,7 @@ impl<
 				target_para_id.into(),
 				included_head,
 				slot_digest,
+				slot_durations,
 			);
 		}
 
@@ -493,12 +499,12 @@ pub fn mock_open_hrmp_channel<
 	recipient: ParaId,
 	included_head: HeaderFor<C>,
 	mut slot_digest: &[u8],
+	slot_durations: &SlotDurations,
 ) {
-	const RELAY_CHAIN_SLOT_DURATION: SlotDuration = SlotDuration::from_millis(6000);
 	let slot = Slot::decode(&mut slot_digest).expect("failed to decode digest");
 	// Convert para slot to relay chain.
-	let timestamp = slot.saturating_mul(SLOT_DURATION);
-	let relay_slot = Slot::from_timestamp(timestamp.into(), RELAY_CHAIN_SLOT_DURATION);
+	let timestamp = slot.saturating_mul(slot_durations.para.as_millis());
+	let relay_slot = Slot::from_timestamp(timestamp.into(), slot_durations.relay);
 
 	let n = 1_u32;
 	let mut sproof_builder = RelayStateSproofBuilder {