Make it possible to use `context` in `WorkflowRehearsal` Test Cases

[jpavlav]

I was working on some unit tests for some st2 workflows and the WorkflowRehearsal test case doesn't account for times when the st2 context may be accessed in the workflow. I have a workflow that defines in the vars section a variable that includes st2.action_execution_id. It results in an exception like this one:

E   AssertionError: {'context': [{'type': 'yaql', 'expression'[161 chars]t'}]} != {}
E   + {}
E   - {'context': [{'expression': '<% ctx().st2.action_execution_id %>',
E   -               'message': 'Variable "st2" is referenced before assignment.',
E   -               'schema_path': 'properties.output',
E   -               'spec_path': 'output[1]',
E   -               'type': 'yaql'}]}

It would be useful to be able to pass in context to the WorkflowTestCase spec and then update the WorkflowRehearsal test case to make use of it.