RFR: 7903519: adding jtharness crashonly extension compatibility [v4]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge pull request #1 from openjdk/master
   
   sync with master
 - jtharness implementation had to be redone as an extension, this is a first attempt to trigger the extension, reporting is not ok yet
 - better crashonly description in help
 - adding crashonly help description
 - adding jtharness crashonly extension compatibility

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/164/files
  - new: https://git.openjdk.org/jtreg/pull/164/files/c5b5a7a2..91abee2c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=164&range=03
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=164&range=02-03

  Stats: 56 lines in 16 files changed: 20 ins; 1 del; 35 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