Fix flaky specs in markdown_editor_spec

What does this MR do and why?

Fix flaky specs in markdown_editor_spec

Instead of relying on nextTick and waitForPromises, dispatch and wait for an event that triggers whenever content editor and markdown editors are ready.

Fixed issues:

• Fixes #422857 (closed) (duplicate)
• Fixes #412789 (closed)
• Fixes #412618 (closed)
• Fixes #412413 (closed)
• Fixes #411565 (closed)
• Fixes #404734 (closed)

Screenshots or screen recordings

No UI change

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #422857 (closed)