Provide `normalize_doctree_xml` via `sphinx_design_elements.testing`

[amotl]

Thank you for swiftly resolving compatibility with docutils 0.22 (GH-249, GH-255). We could also use the normalize_doctree_xml helper function in downstream projects instead of using custom crafted workarounds. Maybe relocate it to the sphinx_design_elements.testing module, to ship it with the package, so package consumers can use it right away?