RFR: CODETOOLS-7902906: jcstress: Simplify forked executors

[Aleksey Shipilev]

ForkedMain calls into EmbeddedExecutor and awkwardly passes TestResultCollector lambdas to runnner. We can simplify the whole thing.

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Commit messages:
 - Merge done and result frames
 - Further simplify runners
 - Simplify forked executor

Changes: https://git.openjdk.java.net/jcstress/pull/37/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=37&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902906
  Stats: 314 lines in 9 files changed: 64 ins; 217 del; 33 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/37.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/37/head:pull/37

PR: https://git.openjdk.java.net/jcstress/pull/37