RFR: JDK-8290836 Improve test coverage for XPath functions: String Functions

[Bill Huang]

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).

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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