Use jest instead of Mocha

[romandev]

Reason: The readability is better I think. WDYT?

/cc @yjaeseok @hwanseung

[hwanseung]

i looked around jest website. it looks like more simpler to use better than Mocha.

[yjaeseok]

In the case of jest, we can use the latest technology because it is led by facebook. Also, our project does not use libraries like webstorm or debugger, so it seems to be easy to migrate.

[romandev]

Then, I'll start this work :)