RFR: 8343220: Add test cases to AppContentTest jpackage test

[Alexey Semenyuk]

- Update AppContentTest by adding new test cases as requested in the CR.
- Fix uncovered issue in jpackage: if the same path in the app image is sourced from a file and a directory values of `--app-content` option, it bails out trying to replace the file with the directory and vice versa.

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Commit messages:
 - AppContentTest: passes on all platforms
 - AppContentTest: all tests work on Linux and Windows
 - FileUtils: fix an issue uncovered with updated AppContentTest
 - AppContentTest: almost all test cases from JDK-8343220 work
 - ConfigurationTarget: make it more helpful
 - AppContentTest: rework for better coverage
 - TKit: add assertSymbolicLinkTarget()

Changes: https://git.openjdk.org/jdk/pull/27948/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27948&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343220
  Stats: 673 lines in 4 files changed: 542 ins; 84 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/27948.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27948/head:pull/27948

PR: https://git.openjdk.org/jdk/pull/27948