RFR: WIP - crashonly extension for jtharness - for fastdebug testing

[andrlos]

this is a continuation of https://github.com/openjdk/jtharness/pull/49 with @dbessono

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Commit messages:
 - coreless crashonly extension for jtharness

Changes: https://git.openjdk.org/jtharness/pull/55/files
 Webrev: https://webrevs.openjdk.org/?repo=jtharness&pr=55&range=00
  Stats: 103 lines in 6 files changed: 97 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/jtharness/pull/55.diff
  Fetch: git fetch https://git.openjdk.org/jtharness.git pull/55/head:pull/55

PR: https://git.openjdk.org/jtharness/pull/55