Thanks for the clarification.
It would be great to have the testing framework configuration that you have mentioned: http://issues.gradle.org/browse/GRADLE-2659
Setting the log level on a specific task does not seem to work: http://issues.gradle.org/browse/GRADLE-2658
I guess, for now, running the tests as JUnit tests with Gradle and as ScalaTests from within the IDE might be an acceptable compromise: http://www.scalatest.org/getting_started_with_junit_4_in_scala