The main reason junior engineers/programmers don't take lots of time to design and perform test scripts, is because most CS certifications do not heavily require this, so other areas of engineering are covered further in college programs, such as design patters.
In my experience, the best way to get the junior professionals into the habit, is to make it part of the process explicitly. That is, when estimating the time an iteration should take, the time of design, write and/or execute the cases should be incorporated into this time estimate.
Finally, reviewing the test script design should be part of a design review, and the actual code should be reviewed in the code review. This makes the programmer liable for doing proper testing of each line of code he/she writes, and the senior engineer and peers liable to provide feedback and guidance on the code and test written.
