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

[Alexander Matveev]

- 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.

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Commit messages:
 - 8362598: [macos] Add tests for custom info plist files [v2]
 - 8362598: [macos] Add tests for custom info plist files

Changes: https://git.openjdk.org/jdk/pull/27509/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27509&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8362598
  Stats: 204 lines in 4 files changed: 179 ins; 23 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27509.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27509/head:pull/27509

PR: https://git.openjdk.org/jdk/pull/27509