RFR: 7903217: jtreg could try killing descendants of stuck test, before timing out the test

[Gerard Ziemski]

This is an enhancement that aims to improve the robustness of the testing by attempting to quit any child processes (that are possibly stuck and are blocking the parent process from terminating) before timing out the target parent process.

Aborting a process will flush its stdout/stderr streams, which will hopefully get captured in the test's log and provide additional clues as to why a test was timing out.

This enhancement was locally tested with a handcrafted test that itself launched a child process that would get stuck on purpose and worked as intended.

Hopefully, this will help debug issues such as [JDK-8286345](https://bugs.openjdk.org/browse/JDK-8286345)

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Commit messages:
 - try quitting any child process before timing out the target/parent process

Changes: https://git.openjdk.org/jtreg/pull/97/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=97&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903217
  Stats: 37 lines in 3 files changed: 31 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jtreg/pull/97.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/97/head:pull/97

PR: https://git.openjdk.org/jtreg/pull/97