Fix timeout handler printing jstack for python3

Change-Id: Id4ad2a842c6fc78c985fb477b2f14930945fc3d5
diff --git a/tools/test.py b/tools/test.py
index ec4bb00..5b1a914 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -476,7 +476,7 @@
   return return_code
 
 def print_jstacks():
-  processes = subprocess.check_output(['ps', 'aux'])
+  processes = subprocess.check_output(['ps', 'aux']).decode('utf-8')
   for l in processes.splitlines():
     if 'art' in l or 'dalvik' in l:
       print('Running art of dalvik process: \n%s' % l)