extracted useful code from #2982
This PR:
- adds test 2 for Rococo <> Westend bridge: checks that relayer doesn't
submit any extra headers while there are no any messages;
- adds test 3 for Rococo <> Westend bridge: checks that relayer doesn't
submit any extra headers when there are messages;
- fixes most of comments from #2439 (like: log names, ability to run
specify test number when calling `run-tests.sh`).
Right now of all our tests, only test 2 is working (until BHs will be
upgraded to use async backing), so you can test it with
`./bridges/zombienet/run-tests.sh --test 2` locally.
Svyatoslav Nikolsky