testing: add richer assetions

[josephjclark]

In tests I want to be able to:

• call a webhook and assert on the response (sync or async). Eg lightning.workflow('x').respondWith(code, payload)
• trigger a workflow and assert on the exist state lightning.workflow('x').exitWith('success')
• trigger a workflow and get the output for a steplightning.workflow('x').output('step-a', expected)
• trigger a workflow and get the final state lightning.workflow('x').finalState('step-a', expected)