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

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.

Start with the WorkflowTestCase spec and the WorkflowRehearsal test case, then trace how workflow variables are evaluated when st2.action_execution_id is referenced. Add support for passing context through the spec and using it during rehearsal, and verify that the reported context assertion no longer occurs for workflows using st2 context.