irmin: add legacy irmin tests

What

MR on top of !12851 (merged)

This MR adds legacy tests from https://github.com/mirage/irmin/tree/main/test/irmin

Why

In house Irmin should be tested with its legacy tests.

How

Adds the legacy tests in /irmin/test/irmin.

Manually testing the MR

Run the following commands

dune exec irmin/test/irmin/generic-key/main.exe
dune exec irmin/test/irmin/data/main.exe
dune exec irmin/test/irmin/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