arbiter:  fix autostart=False watchers during start

In ebec3e50ec3e804caa9df0275d547e07705deae9 arbiter.start() switched
from using arbiter.start_watcher() to using arbiter.start_watchers().
The later does not respect the autostart attribute for each watcher.
This is a regression from behavior in 0.9.2.

Fixed by checking watcher.autostart in arbiter._start_watchers() and
added a unittest to check this in the future.