RFR: 7903519: adding jtharness crashonly extension compatibility

[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 .

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

Commit messages:
 - adding crashonly help description
 - adding jtharness crashonly extension compatibility

Changes: https://git.openjdk.org/jtreg/pull/164/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=164&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903519
  Stats: 13 lines in 3 files changed: 12 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