Add test for a union using an alias with scope (backport #5330)

[mergify]

Description

Add tests for the regression type for issue #21823 in eProsima/Fast-DDS-Gen#411.

Depends on:

• [21824] Regression test for 21823 dds-types-test#39

@Mergifyio backport 3.1.x 3.0.x

Contributor Checklist

• Commit messages follow the project guidelines.

• The code follows the style guidelines of this project.

• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally

• N/A Any new/modified methods have been properly documented using Doxygen.

• N/A Any new configuration API has an equivalent XML API (with the corresponding XSD extension)

• N/A Changes are backport compatible: they do NOT break ABI nor change library core behavior.

• N/A Changes are API compatible.

• N/A New feature has been added to the versions.md file (if applicable).

• N/A New feature has been documented/Current behavior is correctly described in the documentation.

• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• If this is a critical bug fix, backports to the critical-only supported branches have been requested.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.

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This is an automatic backport of pull request #5330 done by [Mergify](https://mergify.com).