src/sndfile.c : Make WAV/GSM610 with sample rates other than 8kHz invalid.

GSM 6.10 is only valid for 8kHz sample rate. See:

    https://en.wikipedia.org/wiki/Full_Rate

Patch-from: Jan Stary <hans@stare.cz>
tests/ : Fix tests for SF_FORMAT_GSM610.

Commit a0be1713 only allows SF_FORMAT_GSM610 to be used with a sample
rate of 8Khz. Adjust tests to use sample rate of 8000 whenever the
sub-format isf GSM610.
programs/sndfile-convert.c : Improve error message for GSM610.

Suggested-by: Jan Stary <hans@stare.cz>