you should definetly take the exam!!!
each and every test paper have lots to teach .Undergoing exam makes you more comfortable and used to to the exam conditions.You already know that you will not be able to score much due to no preparations but still you can try lots of question solvng tricks during that exam such as elimination ,smart guesses and many more.Use common sense and pure creative thinking to think out of the box and solve problems you have never done.Try solving problems without using formulae.
Questions are basically testing the very fundamentals in every portion as the jee does so have a good look every time you go for exam.The solution set they provide can be used latr for your later revisions.Also you may take the test another time if you have the question paper.
						

							
Hi,
 
 NEVER EVER DROP OR MISS ANY TEST/EXAM. 
This isn't your final exam so why are you even worried about negative score and such stuff? Study as much as possible, try your best. Every mock test is an opportunity to improve that never comes back. Give the test, analyze the paper thoroughlt after giving it. You can solve it at home again once you cover up. And in future, try to keep up with the course. Not giving tests just makes you lazy. I have known many people who dont give tests saying they will attempt the whole paper once they cover the syllabus, and then they keep saying this about every test, backlog keeps piling up, and its too late by the time they realize their mistakes. So dont be lazy now and start studying ;)