Proto/test/AI: check computation of frozen rights

What

In the scenario state, compute the expected frozen rights from the simulated frozen stake, instead of relying on the initial_frozen_deposits RPC. Still, also call the initial_frozen_deposits RPC and check that its result is the same.

Why

To test the computation of the frozen rights from the various stakes.

How

Update the function that computes rights for the scenario state at the end of each cycle. This has highlighted some small inaccuracies in the scenario state, which are fixed in the two subsequent commits.

Manually testing the MR

dune exec src/proto_alpha/lib_protocol/test/integration/main.exe

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