Right now, on an existing mailing, if we enable the a/b testing and there's
already a campaign set, we remove it and add a newly create campaign that
matches the name of current mailing. Also, while duplicating a campaign, the
`ab_testing_completed` field should not be copied over.
This commit improves the behavior and only creates a new campaign if there's
not one already set on a record, and by preventing `ab_testing_completed`
from being copied.
TaskID-2653806
Part-of: odoo/odoo#77768
Dharmraj Jhala
3d8faf9b9d