gsup: Extract the new MSISDN string

Extract the new MSISDN IE from the GSUP message and verify that
it is read/written to the message.
diff --git a/openbsc/tests/gprs/gprs_test.c b/openbsc/tests/gprs/gprs_test.c
index 0ae06e7..a7c2401 100644
--- a/openbsc/tests/gprs/gprs_test.c
+++ b/openbsc/tests/gprs/gprs_test.c
@@ -443,6 +443,8 @@
 	static const uint8_t update_location_res[] = {
 		0x06,
 		TEST_IMSI_IE,
+		0x08, 0x07, /* MSISDN of the subscriber */
+			0x91, 0x94, 0x61, 0x46, 0x32, 0x24, 0x43,
 		0x04, 0x00, /* PDP info complete */
 		0x05, 0x12,
 			0x10, 0x01, 0x01,