Sustainable Drupal Development for Teams

Long-term Drupal projects present unique challenges to development teams.

A few questions I hope to answer:
* How do I ensure that a module is automatically enabled across all environments, including developer's local environments?
* Another team member's work depends on my partially-complete feature. How can we best merge our work? What if we need to deploy to production before a feature is finished?
* We want to write unit tests. Where do we start?
* Our unit test suite takes a long time to run. How do I only run tests that are only relevant to the files that I changed?

Topics:
* Code reviews
* Code linters
* Deployment
* Feature branching
* Feature flags
* Unit testing
* Integration testing
* Automated visual testing
* Configuration Management
* Vagrant
* Anti-patterns

Hoping this will provide some good discussion. I can present solutions to problems I've seen teams face, but would also like to hear how others tacked these problems.