Scoru/Tests: Test some properties of proofs inclusion production

Context

Fixes #2981 (closed)

This MR adds tests that attempt to exhibit the bug(s) reported in !5269 (closed) about inbox history. These tests' titles are:

• Checking inboxes history length
• Checking inboxes history content and order
• Checking inclusion proofs validity depending on history capacity
• Checking results of add_messages when histories have different capacities

The tests exhibit two bugs:

1. A first one shows that the content of the history is not correctly maintained due to an Int64.pred in the code
2. A second one related to a next_index overflow (an int64 counter used to get the key/id of the next entry to add), mainly exhibited when fixing the first bug by naively removing the Int64.pred of 1.

Manually testing the MR

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