What

This MR import the irmin-test lib in the in-house Irmin test helpers.

Why

This library is used by all the Irmin test that needs to be added in the in-house Irmin testsuite.

Copy the source of the library and then adapt the manifest to configure the dune and opam files properly.

First tests to be integrated are in the following MR

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