Tezt: fix bake_for_and_wait when providing a count

What

Fixing bake_for_and_wait when providing a ~count argument to match the expected semantic

Why

One would expect bake_for_and_wait ~count:10 client lwt promise to resume when the node reaches current_level + 10 instead of current_level + 1 currently

How

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