[jdk21u] RFR: 8314120: Add tests for FileDescriptor.sync

[Aleksey Shipilev]

Clean backport to cover `sync` paths.

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Commit messages:
 - Backport 2e8a0ab27227b2e06e2ece3776f66ff0932ef353

Changes: https://git.openjdk.org/jdk21u/pull/213/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=213&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314120
  Stats: 165 lines in 2 files changed: 165 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/213.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/213/head:pull/213

PR: https://git.openjdk.org/jdk21u/pull/213