RFR: JDK-8289949 Improve test coverage for XPath: operators

[Bill Huang]

Provided coverage for XPath operators. Operators include:
* Boolean operators: or, and, =, !=, <, <=, >, >=
* Number operators: +, -, *, div, mod
* Union operator: |

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Commit messages:
 - Created XPathOperatorExpTest.java

Changes: https://git.openjdk.org/jdk/pull/10047/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10047&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289949
  Stats: 141 lines in 1 file changed: 141 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10047.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10047/head:pull/10047

PR: https://git.openjdk.org/jdk/pull/10047