[jdk17u-dev] RFR: 8290836: Improve test coverage for XPath functions: String Functions

[Victor Rudometov]

Backport for 
https://bugs.openjdk.org/browse/JDK-8290836 Improve test coverage for XPath functions: String Functions
https://bugs.openjdk.org/browse/JDK-8290837 Improve test coverage for XPath functions: Boolean Functions
https://bugs.openjdk.org/browse/JDK-8290838 Improve test coverage for XPath functions: Number Functions

cherry picked from commit [d889264c6123b6c28bdd6336f5ae547e4e441aa7 ()](https://github.com/openjdk/jdk/commit/d889264c6123b6c28bdd6336f5ae547e4e441aa7)

Clean backport, parity with 17.0.8-oracle
Low risk, test update and new test

Test passes on linux, macOS

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Commit messages:
 - 8290836: Improve test coverage for XPath functions: String Functions

Changes: https://git.openjdk.org/jdk17u-dev/pull/1143/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1143&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290836
  Stats: 821 lines in 5 files changed: 806 ins; 3 del; 12 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1143.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/1143/head:pull/1143

PR: https://git.openjdk.org/jdk17u-dev/pull/1143