cumulus: fix test runtimes panic (#2039)

the min slot duration should be 0 only if the `experimental` feature is
enabled. otherwise, the runtime will panic on a division by 0.

substrate/frame/aura/src/lib.rs

@@ -408,8 +408,9 @@ impl<T: Config> OnTimestampSet<T::Moment> for Pallet<T> {
 		let timestamp_slot = moment / slot_duration;
 		let timestamp_slot = Slot::from(timestamp_slot.saturated_into::<u64>());
 
-		assert!(
-			CurrentSlot::<T>::get() == timestamp_slot,
+		assert_eq!(
+			CurrentSlot::<T>::get(),
+			timestamp_slot,
 			"Timestamp slot must match `CurrentSlot`"
 		);
 	}