Use six.text_type instead of unicode function in tests

This patch replaces unicode function with six.text_type in tests.
Let test code don't break in Python3 environment.
See http://pythonhosted.org/six/#six.text_type

Change-Id: I27e0db2034da7898f713719cc81f1b6c1f1fdeea
Merge "Use six.text_type instead of unicode function in tests"