test_pam_responder: Fix unicore error
    
    Use raw strings instead; other alternative would be to escepe backslash
    
    E     File "/home/build/sssd/src/tests/intg/test_pam_responder.py", line 647
    E       assert err.find("pam_authenticate for user [auth_only\user1]: " +
    E                      ^
    E   SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 36-37: truncated \uXXXX escape