RFR: 8342576: [macos] AppContentTest still fails after JDK-8341443 for same reason on older macOS versions

[Alexey Semenyuk]

Change suffixes produced by `TKit.createUniqueFileName()` function from `.<NUM>` to `-<NUM>` (e.g.: from `.0` to `-0`).
Apple's `codesign` tool can't sign directories with periods in their names as it interprets them as bundles. Directories produced by `TKit.createUniqueFileName()` may end up in an app image and become a subject for signing if used with `--app-content` jpackage parameter

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Commit messages:
 - 8342576: [macos] AppContentTest still fails after JDK-8341443 for same reason on older macOS versions

Changes: https://git.openjdk.org/jdk/pull/21741/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21741&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342576
  Stats: 14 lines in 2 files changed: 2 ins; 9 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21741.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21741/head:pull/21741

PR: https://git.openjdk.org/jdk/pull/21741