As announced on the Gradle blog we are planning to completely migrate issues from JIRA to GitHub.

We intend to prioritize issues that are actionable and impactful while working more closely with the community. Many of our JIRA issues are inactionable or irrelevant. We would like to request your help to ensure we can appropriately prioritize JIRA issues you’ve contributed to.

Please confirm that you still advocate for your JIRA issue before December 10th, 2016 by:

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We look forward to collaborating with you more closely on GitHub. Thank you for your contribution to Gradle!

The following code prints the right directory for me.

apply plugin: 'java'

task printTestClassesDir {
    doLast {
        println test.testClassesDir
    }
}

Seems to be fixed. The property in the DSL also points out the right location.