Disable intermittent markdown serializer test

[m-akinc]

Pull Request

🤨 Rationale

Test failed in this PR build. Following intermittent test protocol.

[test-concurrent:nimble-components] [test-firefox:verbose]   Markdown serializer
[test-concurrent:nimble-components] [test-firefox:verbose]     Serialize rich text editor content to its respective markdown
[test-concurrent:nimble-components] [test-firefox:verbose]       ✗ Mention node (336ms)
[test-concurrent:nimble-components] [test-firefox:verbose] 	InvalidStateError: Selection.collapseToEnd: No selection range exists thrown

👩‍💻 Implementation

Disabled test and linked to existing issue #2219

🧪 Testing

N/A

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.