Add test_args Key to Specify rasa test Arguments (e.g. --domain domain)

[blakemjones]

Locally, I run rasa test --domain domain to test my bot with the split out domain files. However, there is no specified key in the Input Arguments for this action to add arguments directly to rasa test.

I would like another key, perhaps called test_args, that allows me to directly add any arguments to the rasa test command directly rather than just rasa test nlu (test_nlu_args) or rasa test core (test_core_args).