DERBY-6736: The jacoco-junit target fails to store coverage data for some sub-suites

Set the jacoco.active property to the empty string when the tests run
under JaCoCo. The code that did this was removed by mistake in
DERBY-6715.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1625037 13f79535-47bb-0310-9956-ffa450edef68