set all existing tests to -1 for a new student.

[c-harding]

addStudent: Now iterates through test IDs from the testNames array, and sets the new student's result for each test to -1.

[raehik]

Obsoleted by new class Student (addStudent is a lot simpler now). Student.initialise() sets all of the student's results to -1.
(Sorry I didn't put much/any detail in my commits: I'll write up some documentation for the Student and TestDatabase API and maybe throw it in a README.md by Tuesday night.)