RFR: JDK-8315213: java/lang/ProcessHandle/TreeTest.java test enhance output of children

[Matthias Baesken]

We have some failures in TreeTest.java where the expected number of child processes is differing from what we really get. It would be good to have more output to analyze these cases.

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Commit messages:
 - JDK-8315213

Changes: https://git.openjdk.org/jdk/pull/15466/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15466&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315213
  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15466.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15466/head:pull/15466

PR: https://git.openjdk.org/jdk/pull/15466