[jdk17u-dev] RFR: 8290499: new File(parent, "/") breaks normalization – creates File with slash at the end

[Roland Mesde]

Backporting JDK-8290499: New File(parent, "/") breaks normalization – creates File with slash at the end. Fixes the file separator logic and updates a test. Ran GHA Sanity Checks (passed), and related file tests (passed). Patch is clean.

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Commit messages:
 - Backport 21af8bae38dd845d5cbeb2b2770e537b5b9bad22

Changes: https://git.openjdk.org/jdk17u-dev/pull/4175/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=4175&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290499
  Stats: 34 lines in 3 files changed: 28 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/4175.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/4175/head:pull/4175

PR: https://git.openjdk.org/jdk17u-dev/pull/4175