irmin/test: add irmin-mem tests

What

This MR adds the test for irmin-mem library from https://github.com/mirage/irmin/tree/main/test/irmin-mem

This MR depends on !12773 (merged)

Why

Irmin-mem component should be tested

How

By integrating the Irmin-mem test in the in-house Irmin testsuite.

Manually testing the MR

Run the following command:

dune exec irmin/test/irmin-mem/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