RFR: 8343875: Minor improvements of jpackage test library

[Alexey Semenyuk]

The PR affects jpackage test library only. Changes:

- Removed dependencies on `java.io.File`. `java.io.File.pathSeparator` is still used as there is no better alternative.
- Use `TKit.concatMessages()` instead of `String.format()` in TKit.assert...() methods.
- Added unit tests for all TKit.assert...() modified methods.
- Added unit tests for `JavaAppDesc` class.
- Added `jdk.jpackage.test.TestSuite` class to automatically run all jpackage test library unit tests. This eliminates the need to add jtreg comments to every jpackage test library unit test java source.

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Commit messages:
 - Bugfix
 - Extra empty lines removed
 - Blessed modifier order
 - Copyright year and trailing whitespaces fixed
 - Add unit tests for changes in TKit class.
 - Merge branch 'master' into java.io.File-cleanup
 - Bad merge fix
 - Merge branch 'master' into JDK-8343396
 - jpackag tests minor cleanup: use java.internal.util.OperatingSystem; stop using java.io.File

Changes: https://git.openjdk.org/jdk/pull/21995/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21995&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343875
  Stats: 536 lines in 11 files changed: 473 ins; 30 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/21995.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21995/head:pull/21995

PR: https://git.openjdk.org/jdk/pull/21995