Renamed gsm_7bit_ussd() to test_7bit_ussd() and extended the function
to take the expected binary encoding and eventually added trailing
bytes in the re-decoded text as arguments. These are used to check
assertions of the right behaviour instead of solely relying on
regression data, because the value are determined by the spec and
fixed and it is more obvious this way. Especially concerning the case
with the duplicated \r which can easily be overlooked when it's only
present in the ok file.
8a1666b1bf