RFR: 8291650: Add delay to ClassUnloadEventTest before exiting to give time for JVM to send all events before VMDeath

[Chris Plummer]

Near the end of the test, the debuggee forces a GC so ClassUnloadEvents will be generated, and then the debuggee immediately exits. However, the ClassUnloadEvents might not be sent out before the VMDeath is generated, and it prevents delivery of any more events. A short delay after the GC should give the JVM time to deliver the ClassUnloadEvents first.

Note, I was not able to reproduce this issue, so I'm not 100% sure this will fix it. It does turn up in CI testing, so this seems worth trying to see if we still see it during CI.

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Commit messages:
 - Add short delay to give chance to deliver ClassUnloadEvents before VMDeathEvent is generated

Changes: https://git.openjdk.org/jdk/pull/9708/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9708&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291650
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9708.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9708/head:pull/9708

PR: https://git.openjdk.org/jdk/pull/9708