How do you get your diabetic teen (age 19) to comply with testing and taking insulin?

Depends. Find out the reasons for noncompliance and work through them. Sometimes you need a counselor to help.
Time to seek help. The teen years are a minefield of changing emotions & their ability to reason is clouded by alternating feelings of weakness & invulnerability.Counseling may help teach the teen some reasoning skills, separated from their need to rebel against authority. A trip to the local casket shop has had some shock value to some & can open the door to discussion.Discuss it with the therapist before trying.