RFR: 8259518: Fixes for rare test failures due to 8246585: ForkJoin updates

[Martin Buchholz]

8259518: Fixes for rare test failures due to 8246585: ForkJoin updates

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Commit messages:
 - JDK-8259518

Changes: https://git.openjdk.java.net/jdk/pull/2015/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2015&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259518
  Stats: 30 lines in 2 files changed: 22 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2015.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2015/head:pull/2015

PR: https://git.openjdk.java.net/jdk/pull/2015