DERBY-5840: Clean up compiler warnings introduced by using Java 5 language features

Clean up warnings in jdbcapi test suite, and some classes that the
jdbcapi tests depend on in the upgrade test suite.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1483202 13f79535-47bb-0310-9956-ffa450edef68