RFR: 8342544: [macos] jpackage test helper should check for both "--app-image" and "--mac-sign" for signing predefined app image case

[Alexander Matveev]

- Added check for "--mac-sign" argument in cases when we do in-place signing of predefined app image to avoid potential bugs.

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Commit messages:
 - 8342544: [macos] jpackage test helper should check for both "--app-image" and "--mac-sign" for signing predefined app image case

Changes: https://git.openjdk.org/jdk/pull/21776/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21776&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342544
  Stats: 8 lines in 1 file changed: 2 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/21776.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21776/head:pull/21776

PR: https://git.openjdk.org/jdk/pull/21776