RFR: 8287798: Reduce runtime in java.lang.reflect/runtime microbenchmarks

[Claes Redestad]

Add explicit run configurations to java.lang.reflect and .runtime micros, aiming to reduce runtime while maintaining a decently high confidence that there's enough warmup to produce good enough data.

Roughly halves runtime of running `java.lang.reflect|java.lang.runtime` from 159 to 78 minutes.

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Commit messages:
 - Reduce runtime in java.lang.reflect/runtime microbenchmarks

Changes: https://git.openjdk.java.net/jdk/pull/9013/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9013&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287798
  Stats: 27 lines in 4 files changed: 25 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9013.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9013/head:pull/9013

PR: https://git.openjdk.java.net/jdk/pull/9013