Tezt: add migration test with denunciations

What

Add a migration test related to denunciations.

Why

The main point is to increase confidence in the storage stitching when the denunciation- and slashing-related tables are populated. This is also a good occasion to test the new semantics in P for duplicate denunciations wrt rounds and operation kinds, and the special cases of this semantics right after the migration.

How

In tezt/tests/protocol_migration.ml because it already has the tools to trigger the migration.

It would be nice to use double bakings too, to have lower slashing values, but operation_core.ml only supports crafting double operation evidences at the moment. There are older tools for double baking operation, but they are less streamlined.

Manually testing the MR

dune exec tezt/tests/main.exe -- protocol migration double -i

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR