[jdk21u-dev] RFR: 8355475: UNCTest should use an existing UNC path

[Goetz Lindenmaier]

I backport this for parity with 21.0.9-oracle

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Commit messages:
 - Backport e6cea4025b6743538da76f056fa831b02705f423

Changes: https://git.openjdk.org/jdk21u-dev/pull/1957/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1957&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355475
  Stats: 34 lines in 1 file changed: 30 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1957.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1957/head:pull/1957

PR: https://git.openjdk.org/jdk21u-dev/pull/1957