It's always good to frame the benefits of something in terms of things they already care about. Testing is huge for having confidence in making future changes, long-lasting iteration speed, etc. Frame things in terms of what already motivates them.
I'll also add that automated static analysis tools which grade you on certain testing metrics can be great at encouraging people to actually follow through. If you're not already using something like this, I'd recommend it. We use Code Climate. They can be integrated into your git flow and really help maintain accountability and also make improvements more fun. Metrics can and will be over-optimized for so they're a supplement, not a replacement to, good old fashion clean coding.
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
It's always good to frame the benefits of something in terms of things they already care about. Testing is huge for having confidence in making future changes, long-lasting iteration speed, etc. Frame things in terms of what already motivates them.
I'll also add that automated static analysis tools which grade you on certain testing metrics can be great at encouraging people to actually follow through. If you're not already using something like this, I'd recommend it. We use Code Climate. They can be integrated into your git flow and really help maintain accountability and also make improvements more fun. Metrics can and will be over-optimized for so they're a supplement, not a replacement to, good old fashion clean coding.