RFR: 8342609: Bug in jpackage test helper

[Alexey Semenyuk]

`Files.walk(root, 0)` returns the `root` itself and not the contents of the `root`.
Replaced `Files.walk(root, 0)` with `Files.list(root)` to get a non-recursive listing of the `root` directory.

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Commit messages:
 - 8342609: Bug in jpackage test helper

Changes: https://git.openjdk.org/jdk/pull/21582/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21582&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342609
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21582.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21582/head:pull/21582

PR: https://git.openjdk.org/jdk/pull/21582