Added tests to student_test and changed t_shirt size list

I finished some of the tests in student_test.rb (allergies, medications, security_questions, security_responses, etc.)
Also edited the t_shirt size list in registration because the keys and values were reversed

*NOTE* For some reason, whenever I make a pull request, it says that there are more files changed than there actually are. This happened last time I made a pull request and I closed it because it didn't really have much value and everything I did there was redid here. I don't want it to be merged in case any major issues arise.