Configure Groovy compile tasks if present.

This is a first draft to show how integrating the `groovy-base` plugin with `android-test` plugin is completely possible.

I included the runner from [Polidea/RoboSpock](https://github.com/Polidea/RoboSpock) to show that even Robolectic + Spock is a viable option, but I'm not sure if you want to merge that in your example.

I slightly modified some calls in the `AndroidTestPlugin` to show that the configuration of the source directories and compile task could be easily parametrized passing a `String language`, but I left the explicit `if`s to avoid changing too much code.