Bug 1386407 - Part 2: Use prefs.json when testing locally. r=bc

We specify '--preferences' in `awsy_script.py`, but weren't doing so for
local testing. This uses 'conf/prefs.json' by default, but lets the user
override that by specifying '--preferences'.

MozReview-Commit-ID: 5j7a1LA4e8a

testing/awsy/mach_commands.py

@@ -120,6 +120,10 @@ class MachCommands(MachCommandBase):
                 page_load_test_dir]}
             self.run_process(**unzip_args)
 
+        # If '--preferences' was not specified supply our default set.
+        if not kwargs['prefs_files']:
+            kwargs['prefs_files'] = [os.path.join(awsy_source_dir, 'conf', 'prefs.json')]
+
         for k, v in kwargs.iteritems():
             setattr(args, k, v)