Javascript FizzBuzz and Behavior-Driven Development Using Mocha and Chai
Handling invalid input
Instead of returning an array when inputs are invalid, throw exceptions and use throw to detect it in the tests.
Avoid else when previous blocks have return statements
The else can be eliminated since the block evaluated when value &lt;= 0 has a return statement (or would throw an exception). Additionally, the switch statement can be simplified to a few if statements (for an example see this answer to a related question).
Don't forget to set scopes
While the code is in a module, it is best to set the scope of variables and functions - e.g. numbers and i in the loop within numbers should be prefixed by the keywords const and let, respectively, to avoid accidental re-assignment/confusion.
In that same vein, the first few lines of the test are setting variables:

var chai = require('chai');
    expect = chai.expect,
    fizzbuzz = require('../lib/fizzbuzz');


Perhaps you meant to add a comma instead of semi-colon after require('chai')? It would be simpler just to have each variable on its own line:
const chai = require('chai');
const expect = chai.expect;
const fizzbuzz = require('../lib/fizzbuzz');

Actually instead of importing chai just to use expect from it on a separate line, use object-destructuring:
const { expect } = require('chai');
const fizzbuzz = require('../lib/fizzbuzz');