DERBY-534: Support use of the WHEN clause in CREATE TRIGGER statements

Add tests to verify that the when clause operates with the privileges
of the user that created the trigger, and that exceptions thrown in
the WHEN clause are handled gracefully.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1531279 13f79535-47bb-0310-9956-ffa450edef68