Be wary of over-engineered test code.
Excess inheritance, mixins or traits, and other clever tricks to "make the tests simpler" are often a symptom of design issues with the application code.
Focus on your business code's design, and the tests will follow.
A very close runner-up: a test suite that is so over-engineered with inheritance/mixins/... that itself needs to be untied before being able to substantially change the code it's testing.
"Yes but DRY!" - Please spend time on designing your application code, not your tests.