Feature: Gradle Plugin samples runner

[eriwen]

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.

[britter]

Bonus if this also provides a way to have the samples in a different (documentation) module of the build. This is currently not easily possible and requires some boilerplate and has some limitations. I've done it in one of my pet projects:

• In the plugin module publish the pluginUnderTestMetadata as a variant.
• In the documentation module consume the pluginUnderTestMetadata variant

Limitations with this approach:

• Does only work for a single plugin module. If one documentation module should have sample tests for plugins coming from several modules, the individual pluginUnderTestMetadata files would need to be merged in order to construct the classpath
• Publishing pluginUnderTestMetadata breaks plugin publication (see https://github.com/gradle/gradle/issues/13980)
• The solution is coupled to the way the plugin development plugin works (e.g. the existence and output of the PluginUnderTestMetadata task)