Bug 1496828 - [mochitest] Fix mochitest selftest regression by always instantiating self.extraPrefs, r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D8087

--HG--
extra : moz-landing-system : lando

testing/mochitest/runtests.py

@@ -873,6 +873,7 @@ class MochitestDesktop(object):
         self.mozLogs = None
         self.start_script_kwargs = {}
         self.urlOpts = []
+        self.extraPrefs = {}
 
         if logger_options.get('log'):
             self.log = logger_options['log']