Improve test coverage for XPath functions: [JDK-8290836](https://bugs.openjdk.org/browse/JDK-8290836), [JDK-8290837](https://bugs.openjdk.org/browse/JDK-8290837), [JDK-8290838](https://bugs.openjdk.org/browse/JDK-8290838).
------------- Commit messages: - Added exception test cases for node set functions. - Added exception tests. - Extracted common test codes for xpath exp eval to a new function. - Changes for String, Bollean and Number Functions Changes: https://git.openjdk.org/jdk/pull/9752/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9752&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8290836 Stats: 790 lines in 5 files changed: 775 ins; 3 del; 12 mod Patch: https://git.openjdk.org/jdk/pull/9752.diff Fetch: git fetch https://git.openjdk.org/jdk pull/9752/head:pull/9752 PR: https://git.openjdk.org/jdk/pull/9752
