Some (raw) thoughts on what are the elements that give you a quality and testable code.
It’s not a full list of TODOs but some best practices that should serve you as a good baseline. If you have any suggestions, please feel free to comment.

Tests

If we agree that you must have tests in your project – The next good question is what is a good test?

1. It’s FAST – The tests should run fast in order to encourage developers to use them constantly during the development process.
2. Stable – The test doesn’t break often. You wish to minimize the false-positive ratio as much as you can. This is why you need ‘small’ tests that are encapsulated and give you a clear sign on what is working (or not).
3. Easy to read and understand.
4. Catches Bugs! When a test fails it’s most probably a bug.

Code reviews

Code reviews are a critical part of you pull request process.

1. Here is a good tutorial to Code Reviews with some useful recommendations.
2. Understanding code review from Google testing experts.

Static Analysis

1. You can use a SAAS option like Sonarsource
2.  SpotBugs – Can be part of your build/dev process.
3. There are many other options to use linting extensions/plug-in so you will catch many issues during development time.

More

1. Have a good (~75%) coverage of your code with unit tests. It should be a good coverage to effort ratio.
2. Make sure you have unit tests, integration tests, functional tests, end-to-end tests and it’s all automated and part of your build process.
   You wish to have around 70% unit tests (which are ‘small’ tests that run fast and encapsulated), 20% for integration tests and 10% for UI (or end-to-end) tests.
   
3. When you have an issue with production → You creating a test to cover it in the future.
4. Learn from your (and others!) history. 
5. Build a process of debriefing after each time something happened (or almost happened) and create a test that covers this case for the future.
6. You can test in production but think carefully if it’s the right thing to do for your specific case.
   Can you use Mocks, Stubs?
   

Anything else you wish to add?