[crac] RFR: 8368570: [CRaC] Improve JdwpTransportTest

[Roman Marchenko]

JdwpTransportTest sometimes hangs, so it'd be good to dump Java threads, and create a corefile if possible.

Also improved reading from output stream.

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Commit messages:
 - remove copyright
 - test improvements

Changes: https://git.openjdk.org/crac/pull/265/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=265&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368570
  Stats: 144 lines in 3 files changed: 130 ins; 5 del; 9 mod
  Patch: https://git.openjdk.org/crac/pull/265.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/265/head:pull/265

PR: https://git.openjdk.org/crac/pull/265