We always test our code, specially if we write specific custom modules. When we deploy the application, we set up php-test units that run in order to validate the quality of our code. The best procedure to release code is to create different branches for different tickets and sub tasks so that they can be deployed individually and incrementally in an Agile way.
The code must always be merged through a pull request, and must be reviewed by another developer before the merge. The priority of the Tickets must follow a weekly plan so that each deliverable can be individually estimated, developed and delivered on time.
Test Drupal performances across multiple browsers, scenarios, servers, and collect informations that are very useful to improve user experience. We do automation, functional and php unit tests, write documentation about the requirements, deliverables, planning, making estimates, creating mockups, compare solutions and performance.
How to create a Custom Accordion in Drupal 8 using the Paragraphs Module
If you cannot find a module that does exactly what you need then need to write a custom module. First try to find the same functionality in some existing contributed module. Sometimes it is wise to alter or extend an existing module rather then rewriting the functionality from scratch.