Bridge zombienet tests refactoring (#3260)

Related to https://github.com/paritytech/polkadot-sdk/issues/3242

Reorganizing the bridge zombienet tests in order to:
- separate the environment spawning from the actual tests
- offer better control over the tests and some possibility to
orchestrate them as opposed to running everything from the zndsl file

Only rewrote the asset transfer test using this new "framework". The old
logic and old tests weren't functionally modified or deleted. The plan
is to get feedback on this approach first and if this is agreed upon,
migrate the other 2 tests later in separate PRs and also do other
improvements later.