This was in my opinion the best and quickest source on good testing practice, which often meant change the code being tested:
https://misko.hevery.com/code-reviewers-guide/
Sadly, it looks like he has neglected his site, but the content can still be seen through google cache.
This page still seems to work and contains a much meatier amount of content: http://misko.hevery.com/attachments/Guide-Writing%20Testable...