RFR: 8316657: Support whitebox testing in microbenchmarks

[Magnus Ihse Bursie]

The need to use whitebox-testing (wb.jar) from JMH microbenchmarks has recently arisen.

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Commit messages:
 - 8316657: Support whitebox testing in microbenchmarks

Changes: https://git.openjdk.org/jdk/pull/16996/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16996&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316657
  Stats: 61 lines in 4 files changed: 41 ins; 7 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/16996.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16996/head:pull/16996

PR: https://git.openjdk.org/jdk/pull/16996