RFR: 7903519: adding jtharness crashonly extension compatibility [v2]

[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:

  better crashonly description in help

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/164/files
  - new: https://git.openjdk.org/jtreg/pull/164/files/a70e15c7..724ed42e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=164&range=01
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=164&range=00-01

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