> test->compile;test->test i.e. Put all artifacts (test + compile) from the subproject on my classpath.

All I wanted to have is test classpath of subproject is available for (test-compile) on current project. So to try it [changed this file](https://github.com/apache/spark/blob/master/project/SparkBuild.scala#L147) to your suggested configuration. Not sure if I fully understand, it did not compile. 

But then I think I should also have check for test-jar which leads to "compile->compile;test->test" right ?