Heard from a surgeon who is a baal teshuvah:
“I refrain from performing elective plastic surgery because I feel that I am tampering with Hashem’s creation. Hashem granted us the scientific advances that allow us to treat illness and injury. But to alter the existing because of preference is outside of my domain due to my belief in Hashem.”
Following this logic, past generations that lacked the ability to identify recessive genes required only bitachon in Hashem. Today, we must do the hishtadlus for which He gave us the capability.
I think refraining from testing is foolish, and entering those shidduchim with known incompatibility is not bitachon but sheer idiocy.