[BUILD] Make Saros/Core and Saros/I testable under IntelliJ IDEA

This patch amends the IntelliJ project/module meta data.

In Saros/Core:
 - marks the test sources (test/junit) as such
 - adds the JAR files under test/lib as "module libraries for testing"
 - adds JUnit as "module library for testing"

In Saros/I:
 - adds JUnit as "module library for testing"

Change-Id: Id2549ad1c6c0d5cdf459e0a0c75d3a09b005b5c7
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1575
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>