How to write good automated tests

first of all unit tests are the king. The ratio of unit tests should be at least 80% of the total number of tests. No kidding, that's the goal.
tests should run fast - minute or two in total at most when we reach 1.0.0. If we have mostly unit tests then it should not be a problem.
for integration testing in CI we can use things like - software drivers using solely CPU (Mesa 3D), fake display servers such as XVFB. This will make CI builds repeatable and fast. At the moment we are using Docker image spawn on Circle CI.
test-driven development is the BEST way of implementing Pixiq features. The main advantage of TDD is a good API design and .. yes .. greater chance that the code will work. I know, most people in the game industry don't care. In fact othere industries are not way better.

Manual testing