RFR: 8277659: [TESTBUG] ThreadOnSpinWaitProducerConsumer hangs

[Stuart Monteith]

Fix java/lang/ThreadOnSpinWaitProducerConsumer by waiting for consumer thread to finish before restarting trial method.

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Commit messages:
 - 8277659: [TESTBUG] ThreadOnSpinWaitProducerConsumer hangs

Changes: https://git.openjdk.java.net/jdk/pull/6560/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6560&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277659
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6560.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6560/head:pull/6560

PR: https://git.openjdk.java.net/jdk/pull/6560