On Windows, jpackage tests use "taskkill" command to terminate child processes [1] and a powershell script to get the list of started app launchers [2], coupled with the script output parsing code that caused JDK-8344275.

This process management code was added in JDK-8340311. Simplify it by replacing it with java.lang.Process API.

[1] https://github.com/openjdk/jdk/blob/d433ce52360994be5a88a0bcbf39cbb741b435ec/test/jdk/tools/jpackage/helpers/jdk/jpackage/test/WindowsHelper.java#L310
[2] https://github.com/openjdk/jdk/blob/d433ce52360994be5a88a0bcbf39cbb741b435ec/test/jdk/tools/jpackage/helpers/jdk/jpackage/test/WindowsHelper.java#L328