Add removeStudent(); fix removeTest()

removeTest only used to remove the key-value pair in the tests hashmap,
leaving the interface to remove the students' results for that test.
This commit changes that so that TestDatabase does the student result
removal biz.
Match TestDatabase removeX()s; hack in N/A result

Methods to access TestDatabase's remove{Student, Test} methods.

If student hasn't done test, don't show `-1% (-)` -- print "N/A".
Much more clear.