RFR: 8370126: Improve jpackage signing testing

[Alexander Matveev]

On Fri, 17 Oct 2025 18:45:16 GMT, Alexey Semenyuk <asemenyuk at openjdk.org> wrote:

> - Rework the SigningPackageTwoStepTest test to cover the case of bundling an unsigned predefined app image into a signed .pkg installer; make it verify expected warnings in the jpackage output.
> 
> - All jpackage tests will verify that without "--mac-sign" option, jpackage produces app images with a valid "adhoc" signature.
> 
> Additionally:
> - Fix jpackage to make it emit messages expected in the updated SigningPackageTwoStepTest test.
> - Add helper code for signing testing.
> - Automatically unlock jpackage test keychains in all signing tests.
> - Add a workaround for the `/usr/bin/codesign—-verify` command, which sometimes fails if executed without `sudo`.

Marked as reviewed by almatvee (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27875#pullrequestreview-3358446729