RFR: JDK-8290836 Improve test coverage for XPath functions: String Functions [v2]

[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).

Bill Huang has updated the pull request incrementally with one additional commit since the last revision:

  Added comments and fixed typo.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9752/files
  - new: https://git.openjdk.org/jdk/pull/9752/files/5ab30e1b..9bdb1677

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9752&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9752&range=00-01

  Stats: 137 lines in 5 files changed: 46 ins; 15 del; 76 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