In test code deal with `classesDirs` over `classesDir` as appropriate to Gradle version
Typical error emitted form unittest.
org.ysb33r.gradle.gradletest.ClasspathManifestSpec > Creating a plugin classpath manifest STANDARD_OUT
Gradle now uses separate output directories for each JVM language, but this build assumes a single directory for all classes from a source set. This behaviour has been deprecated and is scheduled to be removed in Gradle 5.0