Build out testing framework for SlashGo Server

[navinpai]

Today, we don't have any tests in SlashGo, nor any way to validate changes. To handle this, for the API endpoints, we should build out a testing framework using supertest, mocha,jest etc.

The goal for this is:

1. Integrating testing libraries into the project
2. Adding a npm test step in package.json
3. Adding some sample tests to show functionality in action.