[21313] Test for union with default annotation with external value

[EduPonz]

Description

This PR adds a test case for a @default annotated union which @default value different from any of the union's cases.

Contributor Checklist

• Commit messages follow the project guidelines.
• N/A: Any new/modified type has been properly reflected on test/feature/dynamic_types/dds_types_tests tests in a Pull Request.
• N/A: Any new/modified type has been properly updated on test/dds/xtypes tests using update_header_and_create_cases.py script in a Pull Request.
• N/A: For any new IDL file, Fast DDS update_generated_code_from_idl.sh script adding a new line in file_needing_output_dir list in a Pull Request.

Reviewer Checklist

• N/A: The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.