RFR: 8332885: Clarify failure_handler self-tests

[Ludvig Janiuk]

Adding commetns to clarify that the failure_handler selftests are intended to be run manually. Ideally this would include a more thorough description of the exepcted outputs, but this is what I have time to add right now.

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Commit messages:
 - 8332885 Clarify failure_handler self-tests

Changes: https://git.openjdk.org/jdk/pull/19393/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19393&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332885
  Stats: 15 lines in 2 files changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19393.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19393/head:pull/19393

PR: https://git.openjdk.org/jdk/pull/19393