Xcm emulator nits (#1649)

# Desription

## Summary 

This PR introduces several nits and tweaks to xcm emulator tests for
system parachains.

## Explanation

**Deduplicate `XcmPallet::send(` with root origin code**
- Introduced `send_transact_to_parachain` which could be easily reuse
for scenarios like _governance call from relay chain to parachain_.

**Refactor `send_transact_sudo_from_relay_to_system_para_works`**
- Test covered just one use-case which was moved to the
`do_force_create_asset_from_relay_to_system_para`, so now we can extend
this test with more _governance-like_ senarios.
- Renamed to
`send_transact_as_superuser_from_relay_to_system_para_works`.

**Remove `send_transact_native_from_relay_to_system_para_fails` test**
- This test and/or description is kind of misleading, because system
paras support Native from relay chain by `RelayChainAsNative` with
correct xcm origin.
- It tested only sending on relay chain which should go directly to the
relay chain unit-tests (does not even need to be in xcm emulator level).

## Future directions

Check restructure parachains integration tests
[issue](https://github.com/paritytech/polkadot-sdk/issues/1389) and [PR
with more TODOs](https://github.com/paritytech/polkadot-sdk/pull/1693

).

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Co-authored-by: Ignacio Palacios <[email protected]>