Add parser tests for automatic text nodes

[monapdx]

MML automatically wraps plain text inside nodes when it appears inside semantic tags.

Add test cases confirming that:

claim
Markdown is unnecessary.
/claim

produces something like:

<claim>
  <text>Markdown is unnecessary.</text>
</claim>

Acceptance criteria:

• Add at least 3 passing tests.
• Include text inside claim, evidence, and caption.
• Make sure ordinary text is not mistaken for a tag.