RFR: crashonly extension for jtharness - for fastdebug testing

[andrlos]

fastdebug jvm build testing is about finding out whether the jvm crashes or not... This means we are only interested in crashes and not regular failures when testing fastdebug jvms. This results in this PR that adds "crashonly" atribute for this exact case to jtharness default test runner. 
The intention is to get this addition to work with jtreg without breaking any other jtharness functionality.

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Commit messages:
 - crashonly extension for jtharness - for fastdebug testing

Changes: https://git.openjdk.org/jtharness/pull/49/files
 Webrev: https://webrevs.openjdk.org/?repo=jtharness&pr=49&range=00
  Stats: 65 lines in 5 files changed: 64 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtharness/pull/49.diff
  Fetch: git fetch https://git.openjdk.org/jtharness.git pull/49/head:pull/49

PR: https://git.openjdk.org/jtharness/pull/49