Add simple NN test

[arisanguinetti]

I'm a noobie on testing, not even sure if what I made is correct. I've created a random NN and saved the initial status. then trained the NN 100 times and compared the output with a stored target NN. I think this was mentioned in issue #53 by @shiffman

[GhostRabbit]

Tests of nn.js is desperatly needed but I fear that running a network for 100 iterations can quickly result in the smell of slow tests. But slow tests are better then no tests!

[xxMrPHDxx]

You should randomize the index to train the NN better probably like :-
let index = Math.random() * training_data.length | 0;