produce-xml-converter.xsl should have unit-level XSpec tests

[galtm]

User Story:

Any Metaschema stakeholder can benefit from behind-the-scenes automated testing of the code. This issue is scoped to template- and function-level tests for the produce-xml-converter.xsl stylesheet when it is used in isolation. (That file is also imported by the JSON-to-XML converter generator, and I'm keeping that out of scope. I anticipate that a separate pull request would test behaviors specific to that starting point.)

Goals:

Passing XSpec tests for the units of code in produce-xml-converter.xsl as a self-contained stylesheet.

Dependencies:

{Describe any previous issues or related work that must be completed to start or complete this issue.}

Acceptance Criteria

• All website and readme documentation affected by the changes in this issue have been updated. Changes to the website can be made in the docs/content directory of your branch.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.

{The items above are general acceptance criteria for all User Stories. Please describe anything else that must be completed for this issue to be considered resolved.}