Manifest: introduce dune_with_test option

Context

Depends on !8637 (merged)

This merge request introduces dune_with_test argument to Manifest.test to specify when this declared test is enabled (always, never or with 64 arch).

This allows tezos-lwt-result-stdlib to be included in octez-libs, which was not possible with opam_with_test because of inconsistencies with the other included libraries.

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