How to reference test classes in a depedent project in Gradle

If you have the following depedency:

    compile project(":some-common")

you can't use `some-common`'s test classes.

Add the following dependency for test classes:

    testCompile project(":some-common").sourceSets.test.output

Reference:
http://stackoverflow.com/questions/5644011/multi-project-test-dependencies-with-gradle