Migrate simple example from Polkadot-sdk repo (#116)

In order to ensure that backward compatibility with existing node
Zombienet, this PR migrates (as a start) a [simple example of a test
from
Polkadot-SDK](https://github.com/paritytech/polkadot-sdk/blob/0a6dfdf973b8e7b669eda6e2ed202fb3549a20b9/polkadot/zombienet_tests/smoke/0001-parachains-smoke-test.toml),
and fixes some bugs in order to make the test work;