DERBY-6699 Error in PredicatePushdownTest.test_predicatePushdown

Patch derby-6699-diagnostics makes the sane build print information about the contents
of the two stacks when the ASSERT fails. Since this error is intermittent it may
help debug the issue.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1616807 13f79535-47bb-0310-9956-ffa450edef68