Tezt: fix migration test

Context

The tezt test about migration did not really checked that the migration happens. It baked less block than the level of user_activated_upgrade and checked that next_protocol = migrate_from where migrate_from is the protocol before the migration.

Manually testing the MR

run migration test

Checklist

• n/a Document the interface of any function added or modified (see the coding guidelines)
• n/a Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• n/a 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