Bridges: emulated tests small nits/improvements (#7322)

This PR includes minor fixes identified during work on the larger PR:
[https://github.com/paritytech/polkadot-sdk/issues/6906](https://github.com/paritytech/polkadot-sdk/issues/6906).

Specifically, this PR removes the use of
`open_bridge_between_asset_hub_rococo_and_asset_hub_westend`, which is
no longer relevant for BridgeHubs, as bridges are now created with
genesis settings. This function was used in the generic
`test_dry_run_transfer_across_pk_bridge` macro, which could cause
compilation issues when used in other contexts (e.g. fellows repo).

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