Etherlink/Tezt: mark 'Deposit before and after migration' as flaky
Context
This MR marks Deposit before and after migration as flaky.
Recently I had multiple pipelines that failed because of this tezt, also quoting Romain on slack:
In #core-engineering-tests, it is indeed identified as the most flaky test of the test suite, with 23 detected failures last week, which is very high.
We need to investigate why it is flaky but in the meantime to avoid disturbing core layer 2 MRs, it made sense to me to mark it as flaky.