RFR: 8371184: Improve jpackage test coverage for "--app-image" option [v3]

[Alexey Semenyuk]

> - Verify that when `--app-image` option is specified, files from the predefined app image are copied verbatim into the output app image (JPackageCommand: add `PREDEFINED_APP_IMAGE_COPY`).
> - Add `LauncherVerifier.launcherDescription()` and use it from LiunuxHelper; extend PropertyFinder class for `LauncherVerifier.launcherDescription()`.
>  - Automatically reapply verifications to launchers in the predefined app image to the command referencing the predefined app image through `--app-image` option. Other changes relate to this update.
> 
> Supplementary:
> - Clean up redundancies after [8351369: [macos] Use --install-dir option with DMG packaging](https://bugs.openjdk.org/browse/JDK-8351369) fix: remove `JPackageCommand.dmgInstallDir`, simplify InstallDirTest test.
>  - AppContentTest.java: fix to make test logs stable.

Alexey Semenyuk has updated the pull request incrementally with two additional commits since the last revision:

 - JPackageCommand: verify contents of the predefined app image copied verbatim using more appropriate TKit.assertSameFileContent()
 - TKit: revert the change

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28118/files
  - new: https://git.openjdk.org/jdk/pull/28118/files/c76e49f2..7bf2fa34

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28118&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28118&range=01-02

  Stats: 109 lines in 2 files changed: 18 ins; 60 del; 31 mod
  Patch: https://git.openjdk.org/jdk/pull/28118.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28118/head:pull/28118

PR: https://git.openjdk.org/jdk/pull/28118