RFR: 8289511: Axes: child [v3]

[Bill Huang]

> This is a subtask of [JDK-8286091](https://bugs.openjdk.org/browse/JDK-8286091) which is aiming to improve XPath expression test coverage. The goal of this subtask is validating the XPath child axis specifier in various ways.

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

  Renamed XPahtExpChildTest.java and moved NPE test cases from invalid test case to zero children test case.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9484/files
  - new: https://git.openjdk.org/jdk/pull/9484/files/31525f8c..9a57de45

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9484&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9484&range=01-02

  Stats: 454 lines in 2 files changed: 237 ins; 217 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9484.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9484/head:pull/9484

PR: https://git.openjdk.org/jdk/pull/9484