She "forgot" to get it signed? I think you need to look at the whole picture, and not focus on the signature alone. It sounds like she did not want you to see the test, and that is the reason she signed for you. By getting caught, however, she has probably learned her lesson well enough to prevent a future problem.

I just found out my fourth grade daughter forged my signature. Her teacher left me a message noting that of course I knew about her demerit. My daughter admitted that she was afraid to show it to me. I told her she will be writing apologies to all involved and I will be giving her a punishment. I haven't had a chance to speak with her teacher yet, but I will be making sure this does not happen again.