Add Testing Guide

[napperley]

With Vert.x there is no testing guide for Kotlin. Currently if Vert.x testing is done using Kotlin then JUnit and vertx-unit are used for testing, which isn't Kotlinic (idiomatic Kotlin). Kotlin has two main testing frameworks which are Kotlinic: Kotlin Test and Spek.

A Kotlin testing guide would use one of the major testing frameworks. Kotlin Test would be the recommended one to use since it is easy to deploy (can use Gradle via Gradle Kotlin DSL), does testing in a Kotlinic way, and supports multiple testing types (known as testing styles). This GitHub repository provides some insight into how to approach Vert.x testing using Kotlin and Kotlin Test.