RFR: 8303703: Add support of execution tests using virtual thread factory jtreg plugin [v4]

[Leonid Mesnik]

> The plugin which support execution of test's main method in separate virtual thread is added.
> The plugin is built as a part of test image and might be used in testing by adding JTREG_TEST_THREAD_FACTORY=Virtual option.

Leonid Mesnik has updated the pull request incrementally with one additional commit since the last revision:

  doc updated.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13432/files
  - new: https://git.openjdk.org/jdk/pull/13432/files/fdba79fa..ec2fa4a9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13432&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13432&range=02-03

  Stats: 13 lines in 2 files changed: 5 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/13432.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13432/head:pull/13432

PR: https://git.openjdk.org/jdk/pull/13432