RFR: 8367552: JCmdTestFileSafety.java fails when run by root user [v2]

[Francesco Andreuzzi]

> I noticed this while running the test in a Docker container. The test tries to call `File.setWriteable(false)` on a directory in which it later tries to override an existing CDS archive. Since the root user is not affected by the missing write permission, the test fails consistently.
> 
> I propose to skip it if we detect that 'root' is running it, should be a trivial change.

Francesco Andreuzzi has updated the pull request incrementally with one additional commit since the last revision:

  review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27261/files
  - new: https://git.openjdk.org/jdk/pull/27261/files/373f0c3f..c7e69cea

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27261&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27261&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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