Fix local testing mode by skipping downstream-only config setting

Ideally we'd better replicate the the upstream config, but this
hopefully moves things forward.

Author: asb

buildbot/osuosl/master/master.cfg

@@ -39,7 +39,8 @@ c['workers'] = config.workers.get_all()
 
 c['protocols'] = {'pb': {'port': "tcp:9990:interface=127.0.0.1" if test_mode else 9990}}
 
-c['ignoreOfflineWorkersTimeout'] = 30 # minutes.
+if not test_mode: # Config key only supported on downstream buildbot fork.
+  c['ignoreOfflineWorkersTimeout'] = 30 # minutes.
 
 ####### CHANGESOURCES