Updated Gradle so IntelliJ can pick up the performance test classes

This change in Gradle adds the per source as a test source root if you generate your idea files using "./gradlew idea".  However, IntelliJ won't be able to run the tests as it does not add the perf dependencies into the IntelliJ settings - this either has to be done manually by the user, or the Gradle build can be updated to add these settings to the IntelliJ settings via XML unpleasantness.