How to unit test a plugin in initscript?

For normal unit tests we would just do something like (assuming Spock)

class MySpec extends Specification {

  def project = ProjectBuilder.builder().build()

  def "some test"() {
      project.apply' my.plugin'

    // Rest of test

But about a plugin that is used in a initscript. In this case it is something that is extending Plugin<Gradle>. How does one get an instance of Gradle to apply this to?