gsm: Introduce API osmo_gsm48_rest_octets_si13_decode

Related: SYS#5358
Change-Id: I74fb0a3afc1ac4aadbfc609b882d929401f790eb
diff --git a/tests/gsm48/rest_octets_test.ok b/tests/gsm48/rest_octets_test.ok
new file mode 100644
index 0000000..54204f2
--- /dev/null
+++ b/tests/gsm48/rest_octets_test.ok
@@ -0,0 +1,2 @@
+si13_encode (20): 90 00 d8 5a 6f c9 e5 84 10 ab 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+si13_decode (71)