Make testing scripts python3.x compatible

Update run_tests/*.py to use six based isomorphisms and print function
from __future__ module.
diff --git a/tools/run_tests/run_tests.py b/tools/run_tests/run_tests.py
index 43b8f81..cba91a9 100755
--- a/tools/run_tests/run_tests.py
+++ b/tools/run_tests/run_tests.py
@@ -54,6 +54,7 @@
 import time
 from six.moves import urllib
 import uuid
+import six
 
 import python_utils.jobset as jobset
 import python_utils.report_utils as report_utils
@@ -771,7 +772,7 @@
         runtime_cmd = ['mono']
 
     specs = []
-    for assembly in tests_by_assembly.iterkeys():
+    for assembly in six.iterkeys(tests_by_assembly):
       assembly_file = 'src/csharp/%s/%s/%s%s' % (assembly,
                                                  assembly_subdir,
                                                  assembly,