swarming: Add server-side implementation for supplemental bot_config

appengine/swarming/test_env_handlers.py

Index: appengine/swarming/test_env_handlers.py
diff --git a/appengine/swarming/test_env_handlers.py b/appengine/swarming/test_env_handlers.py
index 6f0d848ca33a2e3e92273fc8d1e152e89a21aafd..a83f0519883971bd8132caadf5124f573dbb2475 100644
--- a/appengine/swarming/test_env_handlers.py
+++ b/appengine/swarming/test_env_handlers.py
@@ -165,6 +165,9 @@ class AppTestBase(test_case.TestCase):
     self.bot_version = response['bot_version']
     params['version'] = self.bot_version
     params['state']['bot_group_cfg_version'] = response['bot_group_cfg_version']
+    # A bit hackish but fine for unit testing purpose.
+    if response.get('bot_config'):
+      params['bot_config'] = response['bot_config']
     return params
 
   def bot_poll(self, bot='bot1'):