Bug 918970 - display existing thread when sending a sms from contact app

old PR : https://github.com/mozilla-b2g/gaia/pull/16216

We need 2 things :
- spy toView to check we call it with the good viewInfo (with or without threadId, contact...)
- wait on async stuff (promise resolution or rejection etc) to start asserting.

For the second goal, the idea (your idea, actually ;-)) was to make toView return a promise we control. The only variation I did was to really call the old toView, because of the boolean isLocked (set to true at the beginning of activities, and set back to false in toView). I didn't want to set it by hand because some  tests were explicitly testing its value... 
So yeah, it might be a bit hacky :-/ Do you have a better idea?