RFR: 7903519 : jtreg/jtharness is missing features for basic crash testing

[andrlos]

this does not exactly add a crash testing option, however it allows users to define their own resultModifiers that they can force jtharness to use via advertising their resultModifier service in META-INF. I tested it with basic crashOnlyModificator and it works as intended - allows me to correctly skip and pass all the regular failures and only leaves me with crash tests as failures. Perfect for corner cases like JVM testing and such.

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Commit messages:
 - resultModifier allows users to define their own result modifying behavior

Changes: https://git.openjdk.org/jtharness/pull/57/files
 Webrev: https://webrevs.openjdk.org/?repo=jtharness&pr=57&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903519
  Stats: 95 lines in 2 files changed: 94 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtharness/pull/57.diff
  Fetch: git fetch https://git.openjdk.org/jtharness.git pull/57/head:pull/57

PR: https://git.openjdk.org/jtharness/pull/57