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.c b/tests/sms/sms_test.c
index 6df4b62..e48f9a3 100644
--- a/tests/sms/sms_test.c
+++ b/tests/sms/sms_test.c
@@ -209,6 +209,19 @@
 	},
 };
 
+static void test_octet_return()
+{
+	char out[256];
+	int oct, septets;
+
+	printf("Encoding some tests and printing number of septets/octets\n");
+
+	septets = gsm_7bit_encode_oct((uint8_t *) out, "test1234", &oct);
+	printf("SEPTETS: %d OCTETS: %d\n", septets, oct);
+
+	printf("Done\n");
+}
+
 int main(int argc, char** argv)
 {
 	printf("SMS testing\n");
@@ -314,6 +327,8 @@
 		}
 	}
 
+	test_octet_return();
+
 	printf("OK\n");
 	return 0;
 }