Migration testing

[LesnyRumcajs]

Issue summary

Currently, we do migrations' testing completely manually. This may hide potential regressions.

We must devise a strategy to do this automatically, for each supported migration and at least for calibnet. Due to the time it takes for the initial start, this may need to be done externally, like the sync-check.

1. Store pre-migration snapshots in DO
2. For each snapshot, import it and assert the migration is successful.

We can speed it up with some DB caching. Ultimately, it'd be best to spawn our devnet for this purpose, but it may be tricky.

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