Testing is an attitude which brings us to trust results based on the fact that we can validate them. Testing is an approach which allows us to think about how to verify we did the right thing even before starting. Testing is a practice which allows us to write effective tests that can be repeated indefinitely while systematically producing consistent results.
Agile is built around the idea of managing complex projects, recognizing the importance of emerging results and verifying in a very disciplined way the assumptions and hypothesis we make as often and as thoroughly as possible. This means testing everything we do, every day ... So if you are truly Agile, you are living testing in every second of your life!