With the introduction of simpletest the drupal landscape did change a lot. Today nearly every change to the D7 Core is demanded to be tested. As a result we got a core that is more reliable than ever before.
But many devs still don't know much about it. My first core patch was a pain. And as it seemed to be over I was demanded to write a test and nearly quit. But it was suprisingly easy to write and took me to a higher level.
Let me help you and give you an introduction how you can provide tests and make all your work more reliable.
Drupal Module Devs that have none or low experience with simpletest