Fixes articulation and render_cfg tests to be proper pytests

[jtigue-bdai]

Description

Fixes some pytest migration errors for test_articulation and test_render_cfg

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there