Testing plugin code with TestKit

A follow up guide on [Getting Started Writing Gradle Plugins](https://github.com/gradle/guides/issues/9) with the focus on testing plugin code.

- Explains the reasoning and philosophy for wanting to test plugin code
- Make a brief distinction about the different types of tests
- How TestKit fits into the picture
- Uses TestKit for implementing and executing functional tests by continuing the example from the Getting Started Guide.

Start by reading the linked “Getting Started Writing Gradle Plugins” guide and issue 9, then use the issue’s TestKit focus to define the follow-up. Done means a guide that explains testing rationale and test types, shows how TestKit fits, and continues the existing example with functional test implementation and execution.

Written by the indexing model from the issue text.