RFR: CODETOOLS-7903476: JMH: Threading model should not assume stable threads [v3]

[Aleksey Shipilev]

> See the bug for the original bug report.
> 
> This PR makes sure the harness code is able to work with different styles of executors, including the custom executor that always gives us a new thread. Integration tests now run with several important executor modes to catch errors early.

Aleksey Shipilev has updated the pull request incrementally with three additional commits since the last revision:

 - Do not test FJP_COMMON, test FJP instead
 - Wrap more things in privileged blocks
 - Working on proper test matrix

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Changes:
  - all: https://git.openjdk.org/jmh/pull/107/files
  - new: https://git.openjdk.org/jmh/pull/107/files/ba139c90..94db0ac8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmh&pr=107&range=02
 - incr: https://webrevs.openjdk.org/?repo=jmh&pr=107&range=01-02

  Stats: 55 lines in 4 files changed: 40 ins; 4 del; 11 mod
  Patch: https://git.openjdk.org/jmh/pull/107.diff
  Fetch: git fetch https://git.openjdk.org/jmh.git pull/107/head:pull/107

PR: https://git.openjdk.org/jmh/pull/107