As Journeyman Geek has answered there is no Stack Exchange site that has a set of practice tests you can do. However... as I wrote in 2014, I think it's still possible to use Stack Exchange for this purpose.
... if I have one bit of advice for someone who honestly wants to learn
something here it's: answer everything. I do not mean click "Post
Your Answer". I mean attempt to answer everything within a problem
domain that you have some experience.
As you post check everyone else's answer and assess it against your
understanding of the question. If you're completely wrong then it
doesn't matter and you don't need to post. If you're correct, or if
everyone else is completely wrong (it happens!) then post...
You're not doing tests, interview questions or exercises, true. You're investigating and solving real world problems that people are currently facing. You can learn a huge amount this way.
It's likely if you're starting out that someone has posted a better answer than you. That's fine, you have just learnt something. If there's no better answer (and you have working code) you can ask on Code Review for someone to assess how it could be improved. If you want to check whether the testing strategies you're using in your answer are correct there's Software Quality Assurance & Testing.
Personally, I much prefer the real world over contrived examples. YMMV.