RFR: 8375050: Simplify process management in jpackage tests

[Alexey Semenyuk]

Replace "taskkill" and powershell script managing child processes started by jpackage tests on Windows with `java.lang.Process` API.

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Commit messages:
 - Simplify process termination on Windows.

Changes: https://git.openjdk.org/jdk/pull/29172/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29172&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375050
  Stats: 238 lines in 8 files changed: 67 ins; 126 del; 45 mod
  Patch: https://git.openjdk.org/jdk/pull/29172.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29172/head:pull/29172

PR: https://git.openjdk.org/jdk/pull/29172