RFR: 8356219: jpackage places libapplauncher.so in incorrect location in the app image

[Alexey Semenyuk]

- Refactor PathGroup
 - Add test cases to PathGroupTest
 - Update UsrTreeTest to verify "libapplauncher.so" is placed in the correct location in the package
 - Refactor UsrTreeTest to use JPackageCommand assert functions

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Commit messages:
 - Make JPackageCommand.assertFileInAppImage() public. Test location of the "libapplauncher.so"
 - Fix bug in PathGroup that resulted in copying "libapplauncher.so" in wrong location

Changes: https://git.openjdk.org/jdk/pull/25051/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25051&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356219
  Stats: 814 lines in 6 files changed: 641 ins; 64 del; 109 mod
  Patch: https://git.openjdk.org/jdk/pull/25051.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25051/head:pull/25051

PR: https://git.openjdk.org/jdk/pull/25051