Bug 1049916 - Create Firefox Accounts smoke tests to validate user authe...

...ntication R=spenrose
Tried experimenting with various examples provided here: https://www.exratione.com/2013/05/die-child-process-die/  and elsewhere, but having trouble getting them to work as expected.  

I did however remove: this.child.send('stop');  which performs a process.disconnect on the child end. I also replaced this.child.kill()  with  this.child.kill('SIGTERM').  Although these statements are equivalent, I wanted to explicitly call it out.  I think the way I was doing it before was leaving the child process orphaned before I could kill it.  Not sure if you'd prefer to see more error checking here??