Tests: Better libxml2 tests

[asispts]

Description

Follow-up #797 (review)

In this PR, I created tests for each libxml2 version. Since the issue is related to libxml2 library, we can't use @requires extension ... annotation.

Suggested changelog entry

Related issues/external references

Fixes #767

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.

[jrfnl]

@asispts Thank you for your willingness to contribute. Could you explain to me what the added value is of splitting the tests based on the libxml version ?

Edit: note: this PR does not address the remark which is referenced in the description.