test's questions
=> The questions belong to the test.
Ex: This test's questions are more difficult than that test's questions.
 test questions
=> The questions are test questions. That is, the word test describes what kind of questions they are.
Ex: How many test questions did you get right?
Ex: The test questions this term are easy.
Prepositional phrase  questions on the test
=> Questions written on the test itself.
Ex: There will be 50 questions on the test.
Does that help?
Student or Learner