DERBY-6634: Improve test coverage of SqlXmlUtil.java

Add missing test cases for:

- XMLQUERY that returns an atomic boolean

- XMLEXISTS where the query returns an atomic value

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1605287 13f79535-47bb-0310-9956-ffa450edef68