Test Slack in GO

There are a lot of tools out there that do the full-fledged mock (for testing github.com/slack-go/slack), but I still decide to provide an alternative because

1. Instead of mocking slack client, I mock HTTP
2. I only need a (very) few test cases, and I want something very lightweight.

So the steps are:

1. mock the server
2. create a service that will consume the api-client ( slack-go )
3. write test

1. Mock The Server