Reorganize emulated tests folder structure

[franciscoaguirre]

We have different files in asset-hub-westend emulated tests to organize tests.

We have a lot of test cases for cross-chain transfers there that are divided in:

• reserve_transfer.rs
• teleport.rs
• hybrid_transfers.rs

This division is based on the type of cross-chain transfer, with hybrid meaning a mixture of reserve and teleport.
It doesn't really help that much and it's harder to reason about than just doing a division based on the chains involved:

• para_to_system.rs
• system_to_system.rs
• through_ah.rs
• through_relay.rs

All of these could be under a transfers folder to organize them better.

A similar reorganization could be done with bridge hub. One folder per bridge (westend<>rococo and snowbridge) and inside files for different transfers depending on the chains involved.