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/integration-tests/emulated/common/Cargo.toml

@@ -27,6 +27,7 @@ pallet-message-queue = { path = "../../../../../substrate/frame/message-queue" }
 
 # Polkadot
 polkadot-primitives = { path = "../../../../../polkadot/primitives" }
+polkadot-parachain-primitives = { path = "../../../../../polkadot/parachain" }
 polkadot-runtime-parachains = { path = "../../../../../polkadot/runtime/parachains" }
 xcm = { package = "staging-xcm", path = "../../../../../polkadot/xcm" }
 pallet-xcm = { path = "../../../../../polkadot/xcm/pallet-xcm" }