RFR: 8303703: Add support of execution tests using virtual thread factory jtreg plugin [v3]

[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:

  fixed doc

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13432/files
  - new: https://git.openjdk.org/jdk/pull/13432/files/31e3cbd1..fdba79fa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13432&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13432&range=01-02

  Stats: 8 lines in 2 files changed: 3 ins; 0 del; 5 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