For the agile development method the developers used to do their own tests to speedup the process. Specially for a white-box testing the development team itself tests, finds and fixes the issue before it goes ahead. And during black box testing, testers don't bother (neither they need to actually) about the code as they suppose to find corner cases.
But yes white box testing indeed makes the person more aware and efficient.
Writing exhaustive test cases is actually critical to any project development and robustness of the final product. I believe its an art as important as to write code.
RE: How a tester can become a great developer?