Add RunContext test helpers (Fix #498)

[SBoudrias]

Followup complete implementation of the ideas presented in issue #498.

[SBoudrias]

BTW, I didn't wanted to add promises or any extra async handler, so I manage a basic counter to delay the generator from running until async actions ends. It may seems sketchy, but I believe it'll work in this context.

Also, we use #then() method for callback, and it is not related to a promise implementation. Maybe we should rename it to #after() in order to make it less ambiguous. What do you think?

[eddiemonge]

i think a note on where this is relative to would be good

[SBoudrias]

Fixed documentation and tests.

[eddiemonge]

after seems too much like the action that occurs after a test is run, and not after a generator is run.

[SBoudrias]

@eddiemonge Any suggestion as to how we should name it?

[eddiemonge]

I don't know. onEnd onDone generated

[SBoudrias]

Ok, gone with .onEnd()

[addyosmani]

onEnd() WFM too. Thanks for addressing the feedback from the meeting, Simon!