RFR: 8375050: Simplify process management in jpackage tests

[Alexander Matveev]

On Mon, 12 Jan 2026 16:28:30 GMT, Alexey Semenyuk <asemenyuk at openjdk.org> wrote:

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

Looks good.

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Marked as reviewed by almatvee (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/29172#pullrequestreview-3653523755