The tests are fine, your code is not. It fails tests like this one:

assert.strictEqual(longestConsec(["zone", "abigail", "theta", "abigail", "form", "libe", "zas"], 2), "abigailtheta")

It doesn't return the first one.

Count the number of divisors of a positive integer n.

A positive integer is an integer greater than 0.

But if you want to be sure, then yes you could validate the input too. Not needed here tho.