Manifest: Tezt 3.1.0

Context

This just bumps the tezt version required in manifest. We already have tezt 3.1.0 in opam-repos. I'm not really sure this is useful really. The tests themselves do not need Tezt 3.1.0. I think the developers should use the latest version, but if they've done make build-dev-deps then they should already have it.

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