Fix zoom to fix the CI

This PR make some changes to fix the zoom optics device tests on the CI.

• Remove update_on from the controller (which was discussed with @muzelle)
• Clean up the duplicated magnification property setter, and merge it together with _move_motors
• Fix the test values
• Some typo

@mammeri, be careful of the change on update_on. If there is a problem on the beamline, this have to be handled in a different way.