How do you get programmers to build testability support into their code?

The programers should add comment attached with their code

for a white box tester ... as for products the source code will be changed as per requirements and developers or testers who follow them should understand the logic of code easily

Developer has to write the Unit test cases and execute the same or else even they can do Automate the Unit Test cases using JUnit if the Project developed using JAVA technology. that is the proof and green signal to Tester to start the Testing.