RFR: 8362598: [macos] Add tests for custom info plist files

[Alexander Matveev]

On Fri, 26 Sep 2025 02:54:32 GMT, Alexander Matveev <almatvee at openjdk.org> wrote:

> - Added test for custom info plist to cover app image including embedded runtime and runtime installer.
> - Fixed bug in `writePList`. It was missing `writeStartDocument()/writeEndDocument()` and `DOCTYPE` should be full xml string.

@alexeysemenyukoracle Please review.

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PR Comment: https://git.openjdk.org/jdk/pull/27509#issuecomment-3336602592