Clarify the environment:
When developing an external Gradle plugin, it easier to reuse the test platform used inside Gradle build itslef.
The core.gradle build file exposes today a testCompile ivy configuration which is used by most internal Gradle plugins tests.
The attached patch is creating a new Ivy configuration "publicTestCompile" which contains all the spock dependencies used by Gradle tests, and the actual test helper classes present in core test source.
Once this Ivy configuration is exposed, with one line in our gradle file:

testCompile(group: 'org.gradle', name: 'gradle-core', version: '0.9+', configuration: 'publicTestCompile')

We are ready to write spock tests for our plugin.

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Thanks again for reporting this issue. We haven't heard back from you after our inquiry from November 15th. We are closing this issue now. Please create an issue on GitHub if you still feel passionate about getting it resolved.