RFR: 8367552: JCmdTestFileSafety.java fails when ran by root user

[Francesco Andreuzzi]

I noticed this while running the test in a Docker container. Root user ignores write permissions, so the test does not apply.

I propose to skip it if we detect that 'root' is running it.

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Commit messages:
 - check root

Changes: https://git.openjdk.org/jdk/pull/27261/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27261&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367552
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27261.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27261/head:pull/27261

PR: https://git.openjdk.org/jdk/pull/27261