[Test] [Java] Add test example of how to handle x-parent without REF_AS_PARENT_IN_ALLOF normalization

[Mattias-Sehlstedt]

Issue #21799 fixed how the parent is calculated for a model. This can cause issues when using the REF_AS_PARENT_IN_ALLOF normalization setting in order to achieve inheritance in the generated code. See issue #22043 for a discussion on how the issue presents itself.

This PR adds a test to illustrate the issue, and with it also a possible fallback that can be used to achieve the old behavior.

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