RFR: 8311989: Test java/lang/Thread/virtual/Reflection.java timed out

[Alan Bateman]

This is a test only change to a test that exercises core reflection from virtual threads. One of the its tests (testNewInstance6) uses a custom scheduler in setup to check that calling a constructor reflectively, where the constructor parks, doesn't pin the underlying carrier. The test schedules an unpark from a virtual thread but the test is missing a call to Thread::join, thus creating a race between shutdown of the scheduler and the unpark. Two sightings of the test failing so far, both -Xcomp and ZGC.

While in the area, two usages of Thread.sleep in the test have been replaced with latches.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/15073/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15073&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311989
  Stats: 20 lines in 1 file changed: 15 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/15073.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15073/head:pull/15073

PR: https://git.openjdk.org/jdk/pull/15073