gsm0808: Implement helper functions for CONFUSION BSSMAP message decoding.

Also add a test for an actual CONFUSION message parsing.

Change-Id: If8afd2d096fb66c6c2f255a08fc1129de3d09cec
diff --git a/tests/gsm0808/gsm0808_test.ok b/tests/gsm0808/gsm0808_test.ok
index b620e36..eaae7a6 100644
--- a/tests/gsm0808/gsm0808_test.ok
+++ b/tests/gsm0808/gsm0808_test.ok
@@ -910,4 +910,10 @@
   --> gsm0808_cell_id{LAC-CI} = LAC-CI:7777-7777
   --> gsm0808_cell_id{LAI} = LAI:777-007-7777
   --> gsm0808_cell_id{CGI} = CGI:777-007-7777-7777
+Testing decoding CONFUSION
+  Cause class 5/0x5 (Invalid message)
+  Cause 82/0x52 (INFORMATION ELEMENT OR FIELD MISSING)
+  Diagnostics error octet location 0 (Error location not determined)
+  Diagnostics error bit location 15 (Reserved value)
+  Diagnostics message that provoked the error: 00 03 25 03 25 
 Done