test: Support receiving SMS with non-ASCII chars

test: Support receiving SMS with non-ASCII chars

Without this I get the following Python traceback, for an SMS that
contains the UK pound sign.

ERROR:dbus.connection:Exception in handler for D-Bus signal:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/dbus/connection.py", line 230, in maybe_handle_message
    self._handler(*args, **kwargs)
  File "./receive-sms", line 9, in incoming_message
    print("%s" % (message))
UnicodeEncodeError: 'ascii' codec can't encode character u'\xa3' in position 51: ordinal not in range(128)