Find a new job.
OP said (emphasis added):
I have told management this many times but nothing changes, and this type of work is demotivating. Day in, day out, test, test, test.
What am I to do? I am becoming rusty in my actual trade because I rarely do it. All the valuable jobs are absorbed by seniors and all the rubbish they don't want to do I seem to get. I don't mind the company, they don't put much pressure on me, I'm just always bored out of my mind and never doing anything cognitive. Any ideas?
This described my situation 6 yrs ago pretty perfectly. Here's what I did:
I brought a personal laptop to work, found a private place to eat lunch, set up a wifi hotspot from my cell phone, and began doing personal projects for 1 hr each day over lunch. I developed and got better at my actual trade during this time.
I don't mind the company, they don't put much pressure on me
If this is true, you might actually consider increasing those lunch breaks to 1.5 hrs, OR leaving work 1.5 hrs early, OR coming in 1.5 hrs late to work on personal projects during that extra time too. Don't make this permanent, though--the goal is to leave when ready, not stay there working reduced hours and harming your employer by doing so.
I spent every spare second of my evenings, weekends, and holidays developing my skills and doing design projects I really cared about.
I looked for other jobs, left, and got hired at a top tech company in the San Francisco Bay Area. I'm still in this process of developing skills and finding my right fit, but I'm on the right path.
I recommend you do the same: get skills, go interview, get out and move on. Keep in mind each interview is an opportunity to get skills and identify weaknesses too. If you fail an interview, identify why, study like crazy, and have that hole filled prior to the next one. Eventually, you will have filled enough holes you will land a job where you are the DOER, not the TESTER.
The idea of "putting in your time" is nonsense. You don't have to "put in your time" doing something you don't want to do for years. Rather, find a way to do what you really want to do. So long as you are or can become good at it, and can make money at it, go follow your dreams to get a job you are passionate about.