RFR: 8334490: Normalize string with locale invariant `toLowerCase()`

[Naoto Sato]

The test library `jdk.test.lib.Platform` uses no-arg `toLowerCase()` for string comparison, which should be avoided.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/19775/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19775&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334490
  Stats: 7 lines in 1 file changed: 1 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/19775.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19775/head:pull/19775

PR: https://git.openjdk.org/jdk/pull/19775