brassaia: add legacy irmin tests

What

Depends on !12928 (merged).

Backport changes from !12831 (merged) to Brassaia

Why

How

Manually testing the MR

In 2368df4f some assert that fails at running time are introduced to show that the new brassaia tests are Running on the CI. See pipeline https://gitlab.com/tezos/tezos/-/pipelines/1255281353

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