I tried to created a test user for my IAP app for testing. Apple ask for the real credit card number in order to login the test user to apple store. I am afraid that apple store will charge me by chance while I do my testing.
Do you have to provide real credit card number for sandbox user?
No you dont.
Log out by going through: Settings -> Store
Do NOT login with your test user. (Otherwise you get that credit card screen).
Instead run your app and login with the test user when asked to.
Note: If you have already logged in with your test user it will become invalid. So in your case you will need to create a fresh test user and only log in when prompted to do so by your app.
