RFR: 7903373: Add ability to customize test execution using main wrapper plugin [v12]

[Leonid Mesnik]

> The fix adds support of a plugin that customizes test execution.

Leonid Mesnik has updated the pull request incrementally with two additional commits since the last revision:

 - removed spaces
 - updated docs and copyrights

-------------

Changes:
  - all: https://git.openjdk.org/jtreg/pull/136/files
  - new: https://git.openjdk.org/jtreg/pull/136/files/a010c867..beab751c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=136&range=11
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=136&range=10-11

  Stats: 26 lines in 15 files changed: 2 ins; 2 del; 22 mod
  Patch: https://git.openjdk.org/jtreg/pull/136.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/136/head:pull/136

PR: https://git.openjdk.org/jtreg/pull/136