Proto benchmarks: add an intercept to the benchmark of Big_map_mem

Context

The Big_map_mem_const parameter is inferred at one order of magnitude higher than the similar Big_map_get_const and Big_map_get_and_update_const and it is less stable (it dropped by about 33% between nov 25 and nov 29). Looking at the benchmark revealed that it was missing an intercept case. This MR adds it.

Related to !7061 (merged) but these three constants don't seem used in the protocol.

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