tests/rlcmac: Add test to showcase that decode_gsm_ra_cap() fails

Currently code using that function in osmo-pcu is disabled, allegadly
because SGSN was sending incorrect values, but it looks more like a CSN1
issue.

Related: OS#1525, OS#3499
Change-Id: I92c86397f988afaa791871d823a45fa85054f3bb
diff --git a/src/gsm_rlcmac.h b/src/gsm_rlcmac.h
index a4750e8..0ac1207 100644
--- a/src/gsm_rlcmac.h
+++ b/src/gsm_rlcmac.h
@@ -5137,5 +5137,5 @@
  void encode_gsm_rlcmac_uplink(bitvec * vector, RlcMacUplink_t * data);
  void decode_gsm_rlcmac_uplink_data(bitvec * vector, RlcMacUplinkDataBlock_t * data);
  void encode_gsm_rlcmac_downlink_data(bitvec * vector, RlcMacDownlinkDataBlock_t * data);
- void decode_gsm_ra_cap(bitvec * vector, MS_Radio_Access_capability_t * data);
+ int decode_gsm_ra_cap(bitvec * vector, MS_Radio_Access_capability_t * data);
 #endif /* __PACKET_GSM_RLCMAC_H__ */