Add end-to-end conversation tests support to the 2.0 parser

[degiz]

Description of Problem:
The 2.0 YAML Parser should be able to parse the following block:

# End to end testing format:
test_conversations:
- story: A basic end-to-end test
  steps:
  - user: |
     hello
    intent: greet
  - action: utter_ask_howcanhelp
  - user: |
     show me [chinese]{"entity": "cuisine"} restaurants
    intent: inform
  - action: utter_ask_location
  - user: |
     in [Paris]{"entity": "location"}
    intent: inform
  - action: utter_ask_price

On a first look only user key processing is missing from the Stories parser.

Definition of Done:

• Tests are added
• Feature described the docs
• Feature mentioned in the changlog

[tmbo]

branch: conversation-tests

[Archish27]

Hi @tmbo
I have added e2e test for moodbot - #5920

[wochinge]

Is there a PR for this? Found it: #6457