Added unittests for run_slave

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 17041904931791bab095a5261b997b679c65dc36..5abaccdbb8ec7ad659248b9cb0191d27473aec21 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -9,6 +9,7 @@ details on the presubmit API built into git cl.
 """
 
 import contextlib
+import os
 import re
 import sys
 
@@ -127,6 +128,13 @@ def CommonChecks(input_api, output_api):
         path,
         whitelist))
 
+  tests.extend(input_api.canned_checks.GetUnitTestsInDirectory(
+      input_api,
+      output_api,
+      input_api.os_path.join('slave', 'tests'),
+      whitelist,
+      env={'PYTHONPATH': os.pathsep.join(infra_path)}))
+
   with pythonpath(infra_path + sys.path):
     import common.master_cfg_utils  # pylint: disable=F0401
     # Fetch recipe dependencies once in serial so that we don't hit a race