add --no-fail to wav-diff cmd

                cmd = [

                    str(wdiff),

                    "--print-ctest-measurement",

                    # wav-diff return code is 1 if differences are found which

                    # would cause parse_wav_diff to raise an Exception on these cases

                    "--no-fail",

                    str(tmpfile_ref),

                    str(tmpfile_test),

                ]