Create subscriptions predicates

[virgs]

Is your feature request related to a problem? Please describe.
Sometimes, while executing tests, is not unusual to receive not related messages. eg. You're testing a queue subscription while the system is up and running, therefore a lot of not test related messages may be consumed and the expected one ignored.

Describe the solution you'd like
It'd be interesting to explicitly declare which message you're interested. Let's say, I'm interested in messages with HTTP header key: interesting-value. I think this predicate should have the same params as the hooks scripts. Something like:

subscriptions:
    - type: mqtt
       ....
       predicate: JSON.parse(body).something === `interesting`