Using java.util.concurrent in NLSTest instead of handling threads directly.

Using java.util.concurrent in NLSTest instead of handling threads directly.

A test in NLSTest was mixing the "old" and the "new" way of handling
concurrency. This change makes use of the java.util.concurrent facilities to
control concurrency and removes the code that was directly dealing with Thread
objects.

Change-Id: Ie7267776e988a48a5443f0f3fe4eb43e79eee4b1
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>