nat: Parse the id response, extract the IMSI, compare it

Add a test case and also add a basic check that we got some
size checks correct. The next step is to act on the result.
diff --git a/openbsc/tests/bsc-nat/bsc_data.c b/openbsc/tests/bsc-nat/bsc_data.c
index a323979..1bc15c8 100644
--- a/openbsc/tests/bsc-nat/bsc_data.c
+++ b/openbsc/tests/bsc-nat/bsc_data.c
@@ -88,6 +88,14 @@
 0x01, 0x0b, 0x03, 0x01, 0x0a, 0x11, 0x01, 0x00,
 0x15 };
 
+/* identity response */
+static const uint8_t id_resp[] = {
+0x00, 0x15, 0xfd, 0x06, 0x01, 0x1c, 0xdc,
+0x00, 0x01, 0x0e, 0x01, 0x00, 0x0b, 0x05, 0x59,
+0x08, 0x29, 0x40, 0x21, 0x03, 0x07, 0x48, 0x66,
+0x31
+};
+
 /*
  * MGCP messages
  */