RFR: 8292623: Reduce runtime of java.io microbenchmarks

[Eric Caspole]

These changes reduce the run time from about 3h15m to about 2h5m.

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Commit messages:
 - 8292623: Reduce runtime of java.io microbenchmarks

Changes: https://git.openjdk.org/jdk/pull/9942/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9942&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292623
  Stats: 44 lines in 10 files changed: 42 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9942.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9942/head:pull/9942

PR: https://git.openjdk.org/jdk/pull/9942