test_gsup_make_response()
rx = Subscriber-Management OSMO_GSUP_MSGT_UPDATE_LOCATION_REQUEST: imsi="1234567" source_name="incoming-source-name\0" session_id=42 session_state=BEGIN sm_rp_mr=23

writing to an empty struct osmo_gsup_message should populate values as needed:
- args (error=0, final=0)
  Subscriber-Management OSMO_GSUP_MSGT_UPDATE_LOCATION_RESULT: imsi="1234567" destination_name="incoming-source-name\0" session_id=42 session_state=CONTINUE sm_rp_mr=23
  rc = 0
- args (error=0, final=1)
  Subscriber-Management OSMO_GSUP_MSGT_UPDATE_LOCATION_RESULT: imsi="1234567" destination_name="incoming-source-name\0" session_id=42 session_state=END sm_rp_mr=23
  rc = 0
- args (error=1, final=0)
  Subscriber-Management OSMO_GSUP_MSGT_UPDATE_LOCATION_ERROR: imsi="1234567" destination_name="incoming-source-name\0" session_id=42 session_state=CONTINUE sm_rp_mr=23
  rc = 0
- args (error=1, final=1)
  Subscriber-Management OSMO_GSUP_MSGT_UPDATE_LOCATION_ERROR: imsi="1234567" destination_name="incoming-source-name\0" session_id=42 session_state=END sm_rp_mr=23
  rc = 0

writing to an already populated struct osmo_gsup_message, should have no effect:
- args (error=0, final=0)
  Inter-MSC OSMO_GSUP_MSGT_ROUTING_ERROR: imsi="987654321" destination_name="preset-destination-name\0" session_id=11 session_state=END sm_rp_mr=17
  rc = 15
- args (error=0, final=1)
  Inter-MSC OSMO_GSUP_MSGT_ROUTING_ERROR: imsi="987654321" destination_name="preset-destination-name\0" session_id=11 session_state=END sm_rp_mr=17
  rc = 15
- args (error=1, final=0)
  Inter-MSC OSMO_GSUP_MSGT_ROUTING_ERROR: imsi="987654321" destination_name="preset-destination-name\0" session_id=11 session_state=END sm_rp_mr=17
  rc = 15
- args (error=1, final=1)
  Inter-MSC OSMO_GSUP_MSGT_ROUTING_ERROR: imsi="987654321" destination_name="preset-destination-name\0" session_id=11 session_state=END sm_rp_mr=17
  rc = 15
Done.