Replace Markdown training data in tests with YAML training data

[wochinge]

Description of Problem:
Support for Markdown is gonna be removed in Rasa Open Source 3.0. A lot of our unit tests are currently relying on training data format in Markdown format.

Definition of Done:

• All Markdown training data files are replaced with their equivalents in YAML (except if they are needed to test the Markdown readers / writers)
• Unused Markdown files are deleted from the repository