RFR: 8289511: Axes: child [v2]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'openjdk:master' into JDK-8289511
 - Added more XPath expression test cases and created negative tests for invalid expressions.
 - Created XPathChildTest.java

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9484/files
  - new: https://git.openjdk.org/jdk/pull/9484/files/09dbb67b..31525f8c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9484&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9484&range=00-01

  Stats: 83253 lines in 1820 files changed: 41928 ins; 26600 del; 14725 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