Feature: Gradle Plugin samples runner

As a Gradle plugin author, I'd like to inject the Gradle plugin under test into the samples test.

### Expected Behavior
Provide a `GradlePluginSamplesRunner` that configures `withPluginClasspath()` on TestKit.

### Current Behavior

### Context
There's now 2 plugins that want this. Be cool to provide this.

Locate the existing samples test runner and the Gradle TestKit integration, then trace how runners are configured. Add a GradlePluginSamplesRunner that configures withPluginClasspath(); done means the plugin under test is injected into samples tests.

Written by the indexing model from the issue text.