RFR: 8341443: [macos] AppContentTest and SigningOptionsTest failed due to "codesign" does not fails with "--app-content" on macOS 15

[Alexander Matveev]

- `AppContentTest` and `SigningOptionsTest` failed on macOS 15 due to `codesign` no longer fails if additional content is added to app image.
- `SigningOptionsTest` fixed by failing `codesign` due to missing identity, since we only need to check that warning message is displayed when codesign fails and `--app-content` is specified.
- `AppContentTest` is fixed by setting expected exit code based on macOS version.

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Commit messages:
 - 8341443: [macos] AppContentTest and SigningOptionsTest failed due to "codesign" does not fails with "--app-content" on macOS 15

Changes: https://git.openjdk.org/jdk/pull/21313/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21313&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341443
  Stats: 27 lines in 2 files changed: 22 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21313.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21313/head:pull/21313

PR: https://git.openjdk.org/jdk/pull/21313