RFR: 7903519: adding jtharness crashonly extension compatibility [v3]

[andrlos]

> I want to add functionality of https://github.com/openjdk/jtharness/pull/49 to jtreg, which means that this needs to be merged only after the mentioned MR is merged and tested only with a build of https://github.com/andrlos/jtharness .

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

  jtharness implementation had to be redone as an extension, this is a first attempt to trigger the extension, reporting is not ok yet

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/164/files
  - new: https://git.openjdk.org/jtreg/pull/164/files/724ed42e..c5b5a7a2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=164&range=02
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=164&range=01-02

  Stats: 220 lines in 5 files changed: 169 ins; 5 del; 46 mod
  Patch: https://git.openjdk.org/jtreg/pull/164.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/164/head:pull/164

PR: https://git.openjdk.org/jtreg/pull/164