RFR: 8289511: Axes: child

[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.

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Commit messages:
 - Added more XPath expression test cases and created negative tests for invalid expressions.
 - Created XPathChildTest.java

Changes: https://git.openjdk.org/jdk/pull/9484/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9484&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289511
  Stats: 217 lines in 1 file changed: 217 ins; 0 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