chore: removes old example file
refactor(files): tests are cleaner w. test helpers
test(fib): returns null if n is not a counting num
bugfix(multiples): fixed false positives in test

`defined` is not a chai matcher. Use the exist matcher to test whether
the object is null or undefined instead.
test(fib): returns even sum for small size n
solution: the sum of even-valued fibonacci terms
Merge branch 'add/even-fibonacci-numbers'