RFR: 7903373: Add ability to customize test execution using main wrapper plugin [v3]

[Leonid Mesnik]

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

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

  minor cleanup

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/136/files
  - new: https://git.openjdk.org/jtreg/pull/136/files/2af02118..d336515f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=136&range=02
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=136&range=01-02

  Stats: 3 lines in 1 file changed: 1 ins; 2 del; 0 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