RFR: 8314515: java/util/concurrent/SynchronousQueue/Fairness.java failed with "Error: fair=false i=8 j=0"

[Viktor Klang]

While this might not fix 8314515, it should at least make it more exact.

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Commit messages:
 - Improving SynchronousQueue Fairness test to only proceed once observing that the previous thread is enqueued as a producer

Changes: https://git.openjdk.org/jdk/pull/17082/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17082&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314515
  Stats: 23 lines in 1 file changed: 17 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/17082.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17082/head:pull/17082

PR: https://git.openjdk.org/jdk/pull/17082