A "test-taker" is a person who takes a test. Here "taking" a test means doing the test.

What do you call a person who takes an exam?

Word forms: examinees

An examinee is someone who is taking an exam.

What does it mean to be a bad test taker?

Most students who identify as bad test takers simply don't know what standardized tests like the ACT or SAT want from them and struggle with how to approach these tests.