jmx-dev RFR: 8319238: JMX ThreadPoolAccTest.java is too verbose and should fail before timeout

[Kevin Walls]

Discovered while testing changes that made this test fail.  The test failure is hard to diagnose as it logs and retries at full speed, possibly forever, until timeout.  This can hit a log file limit.  We can save thousands of lines of text being printed when the test runs normally and successfully, by waiting half a second before doing the Principal-checking which is the purpose of the test.

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Commit messages:
 - 8319238: JMX ThreadPoolAccTest.java is too verbose and should fail before timeout

Changes: https://git.openjdk.org/jdk/pull/16456/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16456&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319238
  Stats: 26 lines in 1 file changed: 23 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16456/head:pull/16456

PR: https://git.openjdk.org/jdk/pull/16456