AbstractJavaTestFixturesIntegrationTest.groovy

Move test fixtures configuration into a test fixtures plugin
This commit removes the methods from the `JavaPluginExtension` to
two different things:

1. a `java-test-fixtures` plugin, which will do the necessary steps
to create the additional source sets for _producers_ of test fixtures

2. a method on `DependencyHandler` to reference test fixtures, similarly
to what we do for `platform` and `enforcedPlatform`