rasa test should use `TrainingDataImporter`

[wochinge]

Current State
When running rasa train it uses the TrainingDataImporter interface to load and provide data to the underlying functions.
rasa test doesn't use this interface and calls this directly.

Solution
Whenever we import training data we should use the TrainingDataImporter interface.

Todo:

• Make sure rasa test uses the TrainingDataImporter (I believe here and here

[kevinthenet]

I was just going to make this same issue!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[wochinge]

@Imod7 I'll re-open this as the Core part is still missing. Unassigning you though - if you want to complete it you're welcome. Otherwise I'd suggest to finish your Rasa X issue first.