Add test for RcbMarkdown component

[Asymtode712]

Description

Added test for RcbMarkdown component

Closes #123

What change does this PR introduce?

Please select the relevant option(s).

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• CI/CD (updates related to the CI/CD process)
• Documentation update (changes to docs/code comments)
• Chore (miscellaneous tasks that do not fall into the above options)

Checklist:

• The commit message follows our adopted guidelines
• Testing has been done for the change(s) added (for bug fixes/features)
• Relevant comments/docs have been added/updated (for bug fixes/features)

[Asymtode712]

@tjtanjin please review and merge this PR if it's good to go