Improve Penpal runtime + emulated tests (#3543)

Issues addressed in this PR:
- Improve *Penpal* runtime:
- Properly handled received assets. Previously, it treated `(1, Here)`
as the local native currency, whereas it should be treated as a
`ForeignAsset`. This wasn't a great example of standard Parachain
behaviour, as no Parachain treats the system asset as the local
currency.
- Remove `AllowExplicitUnpaidExecutionFrom` the system. Again, this
wasn't a great example of standard Parachain behaviour.
- Move duplicated
`ForeignAssetFeeAsExistentialDepositMultiplierFeeCharger` to
`assets_common` crate.
- Improve emulated tests:
  - Update *Penpal* tests to new runtime.
- To simplify tests, register the reserve transferred, teleported, and
system assets in *Penpal* and *AssetHub* genesis. This saves us from
having to create the assets repeatedly for each test
- Add missing test case:
`reserve_transfer_assets_from_para_to_system_para`.
  - Cleanup.
- Prevent integration tests crates imports from being re-exported, as
they were polluting the `polkadot-sdk` docs.

There is still a test case missing for reserve transfers:
- Reserve transfer of system asset from *Parachain* to *Parachain*
trough *AssetHub*.
- This is not yet possible with `pallet-xcm` due to the reasons
explained in https://github.com/paritytech/polkadot-sdk/pull/3339

---------

Co-authored-by: command-bot <>

cumulus/parachains/runtimes/assets/asset-hub-rococo/tests/tests.rs

@@ -527,12 +527,12 @@ fn test_foreign_asset_xcm_take_first_trader() {
 			let bought = Weight::from_parts(4_000_000_000u64, 0);
 
 			// Lets calculate amount needed
-			let asset_amount_needed =
-			ForeignAssetFeeAsExistentialDepositMultiplierFeeCharger::charge_weight_in_fungibles(
+			let asset_amount_needed
+				= ForeignAssetFeeAsExistentialDepositMultiplierFeeCharger::charge_weight_in_fungibles(
 					foreign_location,
-					bought,
+					bought
 				)
-				.expect("failed to compute");
+			.expect("failed to compute");
 
 			// Lets pay with: asset_amount_needed + asset_amount_extra
 			let asset_amount_extra = 100_u128;

cumulus/parachains/runtimes/assets/asset-hub-westend/tests/tests.rs

@@ -527,12 +527,8 @@ fn test_foreign_asset_xcm_take_first_trader() {
 			let bought = Weight::from_parts(4_000_000_000u64, 0);
 
 			// Lets calculate amount needed
-			let asset_amount_needed =
-			ForeignAssetFeeAsExistentialDepositMultiplierFeeCharger::charge_weight_in_fungibles(
-					foreign_location,
-					bought,
-				)
-				.expect("failed to compute");
+			let asset_amount_needed = ForeignAssetFeeAsExistentialDepositMultiplierFeeCharger::charge_weight_in_fungibles(foreign_location, bought)
+			.expect("failed to compute");
 
 			// Lets pay with: asset_amount_needed + asset_amount_extra
 			let asset_amount_extra = 100_u128;