Add test for a union using an alias with scope

[richiware]

Description

Add tests for the regression type for issue #21823 in eprosima/fast-dds-gen#411.

Depends on:

• eprosima/dds-types-test#39

Contributor Checklist

• [x] Commit messages follow the project guidelines.
• [x] The code follows the style guidelines of this project.
• [x] 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.
• N/A Applicable backports have been included in the description.

Reviewer Checklist

• [x] The PR has a milestone assigned.
• [x] The title and description correctly express the PR's purpose.
• [x] 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.