DERBY-6516 Convert lang/nestedCommit.sql to JUnit NestedCommitTest

Patch derby-6515 converts the test in question to JUnit and wires it
into the lang suite while removing it from the harness tests.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1579762 13f79535-47bb-0310-9956-ffa450edef68