RFR: 8207908: JMXStatusTest.java fails assertion intermittently

[Kevin Walls]

Test failure, reproducible running batches of tests at the same time on the same machine.
The use of "jcmd TestApp ..." gathers more output that the test wants.

Using the PID to match only the wanted process, my batches of 25 tests at the same time run in parallel without failure.

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Commit messages:
 - oops
 - 8207908: JMXStatusTest.java fails assertion intermittently
 - DiagnosticFramework: CmdLine::is_executable() correction

Changes: https://git.openjdk.org/jdk/pull/20037/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20037&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8207908
  Stats: 9 lines in 2 files changed: 1 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20037.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20037/head:pull/20037

PR: https://git.openjdk.org/jdk/pull/20037