Bug 967799. b2gautomation: Wait on the queue instead of polling it. r=ahal

This replaces the stdout property with a getStdoutLines functions that waits on
the queue instead of polling it in a tight loop. This should reduce the amount
of time spent in runreftestb2g.py dramatically.

--HG--
extra : rebase_source : 6e14568285c9269d06848e441f3fa3420e76d6a4