RFR: 7903373: Add ability to customize test execution using main wrapper plugin [v8]

[Leonid Mesnik]

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

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

 - docs updated
 - tests updated
 - actionName removed

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/136/files
  - new: https://git.openjdk.org/jtreg/pull/136/files/8d62ef99..b35eb8c7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=136&range=07
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=136&range=06-07

  Stats: 51 lines in 6 files changed: 18 ins; 24 del; 9 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