encoding: Provide an overload for the gsm_7bit_encode and a simple test

This is required for encoding the SMS header using the alpha numeric
rules.

Reviewed-by: Jacob Erlbeck <jerlbeck@sysmocom.de>
diff --git a/tests/sms/sms_test.ok b/tests/sms/sms_test.ok
index d0e0983..ce6cb17 100644
--- a/tests/sms/sms_test.ok
+++ b/tests/sms/sms_test.ok
@@ -1,2 +1,5 @@
 SMS testing
+Encoding some tests and printing number of septets/octets
+SEPTETS: 8 OCTETS: 7
+Done
 OK