RFR: crashonly extension for jtharness - for fastdebug testing [v6]

[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.

andrlos has updated the pull request incrementally with 10 additional commits since the last revision:

 - crash only extension with minimal core classes modifications
 - 7903545: Unchecked cast in Agent
 - (build) build.number incremented to b27
 - 7903544: Minor tweaks to the Agent source code
 - (build) build.number incremented to b26
 - 7903529: Improve accessibility in JavaTest HTML Reports - add "summary" attribute to the generated tables
 - 7903528: Improve JT HTML reports - quote every attribute value unconditionally
 - (build) build.number incremented to b25
 - 7903527: Improve accessibility in JT HTML reports - every HTML document should have `lang` attribute defined
 - (preview/tck.test) added Assert::assertThrows accepting the specific instance to expect

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Changes:
  - all: https://git.openjdk.org/jtharness/pull/49/files
  - new: https://git.openjdk.org/jtharness/pull/49/files/d5273584..684b50a8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtharness&pr=49&range=05
 - incr: https://webrevs.openjdk.org/?repo=jtharness&pr=49&range=04-05

  Stats: 357 lines in 30 files changed: 241 ins; 98 del; 18 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