The most important skill a senior developer must have is not testing, is not data structures, is not system design, or any fancy design pattern. Crucial skill that secretly all companies are looking for in a senior developer is the ability to deliver, deliver working software, on time, and on budget. Not buggy software, not perfect software. I don't think that exists. All the other skills, the system design, the testing, the fundamentals are just a means to an end.
And the end will always be delivery. If you want to get to senior faster, the number one skill you want to focus on, it's your ability to deliver.