LPS-44580 LinkbackConsumerUtil: tests to document current behavior

This test lives in 'test/unit', but ServiceTestUtil lives in
'test/integration'. So I moved the appropriate 'random' methods to
RandomUtil, which is visible to both and already serves as a
java.util.Random convenience wrapper.

https://issues.liferay.com/browse/LPS-44580
LPS-44580 SF