"Test projects" are never acceptable to me.
If you hire a contractor to refurbish your kitchen... do you expect him to do a "test project" by refurbishing your bathroom first?
If you want a mechanic to tune up your car.... do you expect him/her to complete a "test project" by rotating your tires first?
In my world there is no such thing as a "test project". I get paid for work I do. If a client wants to pay me to complete something, then hire me to complete additional projects, that's fine. But I do not work for free. It makes no difference that the client calls something a "test project" it is still work on my part. The same amount of work if the project were called a "Ren & Stimpy Project" or a "GobbdyGook Project". The amount of effort and time on my part is the same.
The label the client puts on the project means nothing. The reality is, by using "test" they are trying to manipulate into working for free or at a reduced rate because they hope you assume they have more work for you. But who knows if there will ever be anything more for you to do. And by using "test" they can get the work done for free/cheap... and just claim they didn't like it and have nothing more for you. Essentially, "test" is a red flag for a client that will not pay you.
Good clients never ask for "test" projects, ever.
Don't work for free. It's just a way of others to take advantage of you. Your portfolio and work history should be enough to show a prospective client what you are capable of doing.
"Test project" = do something free for us