Chore/Tests: Add tests for new level 4 heading Kramdown behaviour

[MaoShizhong]

Because

4507 changed the HTML structure generated by Kramdown, and existing tests for h3s and sections were adjusted to account for this in that PR. However, there were not any tests that actually tested the new h4 behaviour.

This PR

• Adds tests for Kramdown for the new h4 structure

Issue

N/A

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the keyword: brief description of change format, using one of the following keywords:
  • Feature - adds new or amends existing user-facing behavior
  • Chore - changes that have no user-facing value, refactors, dependency bumps, etc
  • Fix - bug fixes
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] I have verified all tests and linters pass after making these changes.
• [ ] If this PR addresses an open issue, it is linked in the Issue section
• [x] If applicable, this PR includes new or updated automated tests