Even if you failed to complete the testing period there would be no change required on your 2018 Form 8889. Failure to complete the testing period would be handled on the Form 8889 for the year in which the failure occurred, but since you will be telling 2019 TurboTax that you were covered for all of 2019 and that you yourself were not covered by Medicare, TurboTax will not ask anything about your insurance coverage in 2018.
