Support Scalatest

This plugin may be a good basis for supporting Scalatest for Gradle.

https://github.com/maiflai/gradle-scalatest

I didn't want the full functionality of the plugin, just the test action, so I ended up just hijacking that component with the following logic:

```kotlin
tasks.withType().configureEach {
actions = mutableListOf(
com.github.maiflai.ScalaTestAction(null)
) as List>
}
```