They're programming on their own,
Why is this a problem? Are you wanting them to pair program? If so, please rethink ;).
without much interaction with other members
Why is this a problem? If their current programming efforts don't overlap, they don't need to collaborate much.
and they are not interested into testing their code as it was expected to be.
Ah, finally, a concrete problem, and an easy solution.
Add a new step to your process where you code review (as in you specifically). Reject all code that is not tested properly. If this doesn't solve your problem (i.e. stubborn programmers), make it clear that they will be coding to your standards, and if they can't, they need to seek employment elsewhere. Professional software engineering requires professional software engineers.
Seek to engage their sense of pride and professionalism as a psychological ploy if you meet resistance. Most developers long to be taken more seriously and more professionally: show them you want them to be professional, and treat them like professionals, and rapid attitude shifts are possible.