SCORU: add a test for worst case conflict point computation

Context

At @yrg's request, I wrote a small test to check the gas cost of conflict point computation. It seems that in the worst case that can be constructed, the operation exceeds the maximum operation gas limit.

Let's use this MR to discuss if it's an actual issue and/or propose a fix.

Manually testing the MR

dune exec src/proto_alpha/lib_protocol/test/unit/main.exe -- test "^[Unit] Sc_rollup_storage.ml$"

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