Remove duplication in EE/CE markdown_processing_spec

What does this MR do and why?

This MR addresses some duplication between the EE/CE markdown_processing_spec. It also attempts to fix an issue when running the test with something like:

FOCUSED_MARKDOWN_EXAMPLES=label yarn run jest spec/frontend/content_editor/markdown_processing_spec.js

Before	After

MR acceptance checklist

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