RFR: 8291828: Reduce runtime of java.util.stream microbenchmarks

[Eric Caspole]

These changes reduce the default run time from about 1 day 22 hours to about 6 hours with good stability we can use in weekly testing.

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Commit messages:
 - 8291828: Reduce runtime of java.util.stream microbenchmarks

Changes: https://git.openjdk.org/jdk/pull/9733/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9733&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291828
  Stats: 365 lines in 62 files changed: 364 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9733.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9733/head:pull/9733

PR: https://git.openjdk.org/jdk/pull/9733