RFR: 8365555: Cleanup redundancies in jpackage implementation [v3]

[Alexey Semenyuk]

> - Remove AppImageDesc class. It was supposed to bind an AppImageLayout instance with a root directory, but since AppImageLayout has `rootDirectory()` method it is redundant.
>  - Remove redundant `Package.packageLayout()` and `Package.asPackageApplicationLayout()` methods from the model, 
>  - Add tests for PackagingPipeline class.
>  - Move JUnitAdapter.java from "/test/jdk/tools/jpackage/helpers-test" to "/test/jdk/tools/jpackage/junit/tools" directory, support running platform-specific JUnit tests.
>  - Enhance AppImageLayout class, add unit tests for derived MacApplicationLayout and LinuxApplicationLayout classes.
>  - Add tests for BuildEnv class.

Alexey Semenyuk has updated the pull request incrementally with one additional commit since the last revision:

  Fix issues uncovered when running signing tests

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26778/files
  - new: https://git.openjdk.org/jdk/pull/26778/files/9f33d8e2..099da666

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26778&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26778&range=01-02

  Stats: 17 lines in 3 files changed: 8 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/26778.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26778/head:pull/26778

PR: https://git.openjdk.org/jdk/pull/26778